Where can i find the bubble gum jar top for an A-9 burr grinder?

Safety Tip: To prevent any potential mishaps, ensure sure your grinder is fully off and unplugged before using it. Also, if you haven't used it in a while, make sure it's cool to the touch.
Let the Bean Hopper open. Take off the bean hopper from your grinder's top. This is going to reveal the top burr.
Take the Upper Burr Carrier Off: An upper burr carrier is usually used to hold the top burr in place. It is necessary to take out the upper burr carrier. Depending on the kind of your grinder, the precise procedure may change, but generally speaking, it entails undoing a few screws or tabs that secure it. In general, follow these steps:
a. Check the upper burr carrier for any visible screws or tabs. Usually, they're close to the edges.
b. To release the tabs or loosen the screws, use the proper equipment (usually a screwdriver). To prevent breaking the grinder, use gentleness.
c. After the upper burr carrier is released from its locking mechanism, carefully pull or slide it out.
Lift Out the Top Burr: The top burr can now be removed after the upper burr carrier has been taken out. The burr might be secured in place by tiny clips or notches, so proceed with caution and gentleness.
Clean and Maintain: After removing the top burr, you can give the grinder and its parts a thorough cleaning. Make sure you adhere to the cleaning and maintenance guidelines provided by the manufacturer. This often entails cleaning the grinder's parts, brushing off coffee grinds, and correctly reassembling it.
Reassemble: Carefully put the grinder's parts back together in the opposite order after cleaning. Verify that the upper burr carrier is correctly positioned and that all screws and tabs are firmly in place.
Plug in and Test: After the grinder has been put back together, plug it in and make sure it is operating properly.

Question edited for clarity.

Videos and text based articles.

https://www.google.com/search?q=jura+ena+9+grinder+burr+calibration

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I had the same problem, as I had a vintage machine, missing the gasket. (Without the gasket, vibration of machine will cause jar to unscrew by itself.) Put the gasket on the glass, but slide it up on the threads so that it only contacts the machine body when jar is fully screwed in. Jar will indeed get (slightly) difficult to screw on as gasket contacts grinder adjustment ring, but that slight tension is normal and necessary in order for the gasket to achieve it's function.

Left hand threads.

For parts you can see, use an artists paintbrush. If you want to clean the internals, grind some panko breadcrumbs or raw rice grains, then run some coffee through it a few times before using.

The current Kitchenaid A9 burr grinder glass jar is interchangable with the original Hobart/Kitchenaid A9 glass jar; therefor, the gasket should also interchange - contact Kitchenaid customer service to order a gasket.

Setting the grind is accomplished by grasping the body of the ginder in one hand and the grind-selector ring (collar that the jar screws into) in the other. Hold the base tight and turn the selector ring clockwise for a finer grind, counterclockwise for coarser. Be careful not to completely unscrew the selector ring; if you are not sure how much adjustment is left, screw it all the way clockwise until it stops (finest grind); then, back it off, up to one full turn to make the grind coarser.

The selector ring has the words coarse, medium and fine cast into it; these are rough postition markers. There is a triangle cast into the body of the grinder, just above the brand name, that can be used to diral in a preferred level of grind. I use a medium-coarse grind for drip coffee makers; to do likewise, position the selector ring so that the triangle is pointing midway between the words coarse and medium.

I took the jar off and cleaned/scraped the port out(the square hole that the gounds come out of) then gently "bumped" the top and side of the unit on newspapers to get the accumulation of 40 years of stray grounds out and it runs great now!!

It could well be gummed up with years of coffee bean residue

Very simple to take apart. There are two screws holding the cannister. They are hidden under 5mm round rubber covers that you have to pry off. Then remove the two screws holding the plastic cover. Mark the position of the numbered dial relative to the brass burr assembly then remove it. Once exposed, you should be able to figure out why it is locked. Scrape away as much gunk as you can.