Have a shower valve marked upc found cartridge but cant fine other plastic piece that sits below inside of valve body ...what is it called and where is it available...

Moen shower valve bodies can clog internally, and there is a simple fix! 99 times out of 100 you will not need to replace the entire valve.
First remove the shower head and test for flow with the head removed..
Next replace the internal valve cartridge - available for free from Moen.
Next test for flow through the valve body AND through the shower head pipe elbow or arm and the vertical riser piping should be performed with the shower head removed, as those are often clogged.
You can also check for flow through the shower arm and vertical riser above the valve. To do this, twist the elbow so that it points upward. Using a funnel, see if you can pour a glass of water into that elbow and see if it flows nicely out of the front of the valve body (where the cartridge is removed).
On the Moen Positemp Shower Valves you must first remove the cartridge. If replacing the cartridge does not improve the flow, you should verify that you are getting plenty of flow to the valve body. Do this by leaving the cartridge out, stand back, and have someone slowly open the water main until you can verify that there is plenty of flow to the valve body. (Using cell phones makes communication easy) (You can use a towel or rag a few inches from the opening at the valve, as a water deflector to help you stay dry
If there is plenty of flow to the valve and if the cartridge is new, the only thing left is internal clogging of the valve body itself. This can occur over time with sediment accumulating in the tiny orifice connecting the inner diameter of the valve body to the valve outlet to the shower head.
Using a light you can examine the inner surfaces of the valve body while the cartridge is removed. Once you see or feel the roughly 3/16" HOLE in the internal diameter of the brass valve body this opening, and the passage from it is the likely suspect. This 3/16" opening connects internally to the valve outlet to the shower head itself .
The best cleaning tool I have found for the job is a 4" or longer piece of a plastic tie wrap smaller than 3"16 in width. Bend a radius in the end of the tie wrap and insert it into the small hole of orifice in the inside of the valve body. You should be able to slide it inward about 1 1/2" or more. The direction you want the tie wrap to travel is toward the rear or back of the valve body. Work it in and out 3-4 times and your obstruction should be cleared.
Reinstall your cartridge, and the clip, and turn on your water main. You should have much improved flow. After rinsing the shower head line for a minute, you can then reinstall the shower head.Bingo, hopefully you just saved yourself a $500 plumber's bill.( If you can't insert the tie wrap and get about 1 1/2" of insertion then you may have a blockage which may not be removable) (Tie wraps have small ribs or notches in them. When you bend the tie wrap to cause it to enter the 3/16" opening and get the desired 1 1/2"insertion - sometimes the tie wrap will be difficult to remove. This can be due to the ribs of the tie wrap catching on the 3/16" opening. You can slide you index finger inside the valve body and while pushing in on the tie wrap where it enters the 3/16" orifice - and pulling out on the tie wrap at the same time - it should come on out)To all you fine people who replaced the entire valves - sorry about that as it really was not necessary! Good luck to all the rest!
If your Moen Positemp pressure balanced single lever shower valve has Stopped Flowing Almost Completely, even after the cartridge and the shower head were replaced, and if the trick above does not improve the flow, there is a next step you can take if your are a fairly well skilled handy person.
The problem here is likely a very badly clogged valve body that is not serviceable - or at least that's what Moen thinks. So if you can't clear the output with the flexible piece of tie wrap material as described in the prior post, here goes...
.Before you go further, be certain that this is your problem if you haven't done this already: Remove the shower head. Twist the elbow so that it points upward. Using a funnel, see if you can pour a glass full of water into that elbow and see if it flows nicely out of the front of the valve body (where the cartridge is removed). If the water backs up in the elbow, you have a clogged valve body, for sure. Since your water may be off at this point, you can dip a cup into the back of any toilet tank as a water source)
At this point you have identified that you have 2 choices - 1) replace the complete shower valve or 2) perform the following procedure. For most skilled do-it-yourselfers the complete valve replacement is a good half day job, plus the cost of the new valve maybe $150, and you end up with a hole in the wall behind your shower. With my alternative procedure, you should be able to complete it in an hour or so, and the cost of the supplies (except tools) should be under $10.
Tools/material needed:drill, 1/8 drill bit, 7/32 drill bit, teflon tape, 1/4 20 tap, crescent wrench, 1/4-20 set screw or plug (preferably brass or nylon), coat hanger, small compressor, air blow gun attachment
You will notice the round hole in the bottom center inside the valve body. This is the outlet to the shower head. Directly below this hole, and just below the round opening to the valve body, you will see a small squared off area on the outside of the valve body. This squared off section is the passage way or port for the water output to the shower head. The flat front face of this channel measures about 1//2" square, and if you feel the sides of it, you can tell that it extends to the back of the valve body maybe about 1 1/4" or so.
To clear this channel you can drill into the front face of the brass valve casting body with a quality 7/32" steel bit. (You can make a pilot hole using a smaller 1/8" bit first if you like) Next use a 1/4 20 male tap and tap threads into the opening you just created. Using a coat hanger or similar wire, you can thoroughly clean out or dislodge any deposits inside the channel.
Next using compressed air from a compressor and a hand triggered blow off gunt, hold your index finger over the round outlet hole inside the main valve body opening, while at the same time forcing compressed air into the new tapped hold you just created. This air should dislodge any deposits and any water which remain, and they will forcefully exit above at the elbow at the shower head.
Using a good coating of teflon tape install a 1/4- 20 allen screw into the tapped hole, just 3-4 turns or until it's snug. You can also use a 1/4-20 brass or nylon plug if you can find one. To avoid corrosion on the plug or allen screw it's best to use one made of brass or stainless if you can find one. You can also use the threaded end of a nylon 1/4-20 bolt and cut off the head to make a plug if you can't find one. You can add you slot head to the end you cut off using a hack saw. Lowes and most hardware stores have nylon 1/4-20 x 1/2" bolts. Reinsert your cartridge, install the cartridge clip and the plastic knob. Install your shower head. Turn your water on, and bingo - you should have more water than you have seen from this shower in years!!!!
If it works for you , you just saved probably $500 you would have had to pay a plumber! Plus you have the immense satisfaction of saving a shower valve body that you would otherwise have had to trash.</span>