Where do I find a throat insert for a table saw model number 113.299040

have you checked sears website for parts?
https://www.searspartsdirect.com/model/369quo6n6x-000247/craftsman-315218050-table-saw-parts

That is commonly called a throat plate and we need to know what the make and model of the saw is.

Use an appropriate pc of mdf that is same thickness of throat plate already installed. Use a router to flush trim to that throat plate. Now, insert and hold down with clamps and scrap panel. Raise Dado head ,while running, enough to do the job. This completes a zero clearance throat plate. No need to buy!

You could make a tab to replace the original that you can hold in place with a countersunk screw through the top of the table. You are probably going to have to flip the saw over so that you can grind a flat spot where you can mount the new tab. You might even have to cut a gap in one of the cast webs. If you don't want to cut a web, your replacement tab is going to be a bit trickier to make.
Good luck! (I am trying to imagine how on earth the tab broke!) And by the way, it is possible to weld cast iron but I think it would be a mistake to try it because it would almost certainly warp your table.

Those parts are not available. take a look at this article on how to make your own

http://www.rockler.com/articles/zero-clearance-table-saw-inserts.cfm

If you are unable to buy an accessory throat plate from Sears for your saw, you might like to look online at www.manageyourlife.com. You may be able to find accessories there that your local store doesn't stock.
I'm not familiar with your particular saw and it's throat plate, but you should know that many woodworkers make their own inserts using 1/2" plywood cut to fit the shape of the throat plate. You can trace your existing plate onto the form, then carefully cut it out and sand to fit. Insert small screws from beneath the plate to level it in position, level to the table top. Then cut a slot in the new throat plate by raising a small blade up through the new plate. Sometimes I use a skilsaw blade or dado blade to make the first cut. Do this only when it is safely held down with a long board clamped over top of it, while you raise the spinning blade up through the new throat plate to cut a slot or groove, as the case may be. If you can't lower the plate over the blade in its lowest position, you can either use a smaller diameter blade to cut through it, or rest the plate lightly against a blade lowered to its lowest position, then hold the plate in place safely with a long board or piece of plywood that rests lightly against it and holds just enough pressure that you cnn start your saw with the throat plate against the edge of the blade, then exert slight pressure downwards to fit the throat plate fully into the throat, cutting through it into the backing board, then you'll be able to raise the blade through the throat plate. This is a dangerous operation and should only be undertaken with safety equipment such as safety glasses or goggles in place, and alway keep your hands away from the moving blade and exerting force away from the moving blade. Exercise good judgement and caution and you should be able to plunge the new throat plate into position over the moving blade, using the backing board as a safety device/push stick.
If you are unsure about any of these instructions please don't hesitate to ask further, or refrain from taking any actions on your own, and in any event, proceed fully at your own risk.

I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
If I've managed to answer your question or solve a problem, please take just a moment to rate this post....thanks!

Maybe you could try using the outline of the regular insert to cut a new insert out of a piece of polycarbonate (lexan) or ABS plastic. The clips that hold my dado insert barely hold it in place on mine, and I've seen other saws that use plastic.

Narrow throat plate should work. Sure you have the blade installed correctly with arbor washers?

why not just make your own out of 1/4" plywood

trace out the insert but don't cut a slot

install it on the saw with the blade all the way down

turn on the saw and raise the blade to cut out the insert

Adjustments
LARGEHEXWRENCH
TO REPLACeTHE BLADE
See Figures 43 - 45.
 Unplug the saw.
 Lower the saw blade, raise the blade guard, and remove the throat plate.
 Make sure the bevel locking lever is securely pushed to the left. Raise the saw blade to its full height by turning the height/bevel adjusting handwheel clockwise.
 Using the smaller hex wrench, insert the flat open end into the flats on the arbor shaft.
 Insert the closed end of the larger hex wrench over the hex nut. Holding both wrenches firmly, pull the larger wrench forward to the front of the machine. Make sure the blade nut is securely tightened. Do not overtighten.
Check all clearances for free blade rotation. See To Set the Scale to the Blade. In cutting operations, the scale will be set to the side of the blade where the cut will be measured and made.
To replace the blade with an accessory blade, follow the instructions provided with the accessory.
To install a standard blade:
 Place the new blade on the arbor shaft (the teeth must point down toward the front of the saw to work properly).
 Place the blade washer and the blade nut over the blade arbor. Be sure the dome side of the blade washer faces out from the blade and that all items are snug against the arbor housing. Tighten securely.
SMALLHEXWRENCH
Fig. 43
Blade
Bladewasher
arbor shaft
Bladenut
Fig. 44
BLADEGUARD
THROATPLATE
spreader
BLADE
Fig. 4535
ADJUSTMENTS
 Rotate the blade by hand to make sure it turns freely.
 Lower the saw blade and reinstall the throat plate.
NOTE: Securely tighten throat plate screws. Do not allow the throat plate to bow up above the table surface.