Servo cover. Its not a popular leaking point, but it does happen. The "cover" is held in by an internal c-clip of sorts. Theres a tool of sorts that puts pressure on the cover to allow you to release the c-clip and then it gently comes out because theres spring tension behind it. Not a lot of tension, but its there, and you can expect to lose maybe a few ounces of trans fluid when it comes off. Tranny usually doesnt have to come out to replace the part. YOu may just want to take it to tranny shop and have them do it. Its maybe a 1 hour job. The cost of having them do it may offset the time and labor of you having to lift the car, get all greasy and oily, trying to get cover off while not having it shoot into your face if you lose control of the spring tension, and other goodies. Just my five cents worth.
I also found this for you. if you surf down a few pages to "1-2 Accumulator Removal", you can see where they are using a little "ice pick" to put pressure in the cap so you can get access to the c-clip to release the the cover. https://www.cartechbooks.com/techtips/gm-th350-transmission-disassembly-guide/
If you check other suppliers like Oreilly's and Autozone with no luck, call a transmission shop. Most have everything in stock for their own builds and will sell just what you need if they're decent.
https://www.wittrans.com/parts?type=350§ion=13_casevb Not sure of your year. If I was to take it apart id put all new rings in while I have it apart . Not sure where you are but someone is supplying the trans shops in your area. You might google around and see if there is a wit or transtar local to you.
A guy told me about WIT. I called them and have new parts coming Monday. Getting the whole servo assembly. Cap, piston, o rings spring. $30 bucks shipped.