I remember there being an achievement for purchasing a from the Crystal Shop, but I can't find it. Can someone help me out if there is an achievement like that and where to find it? (so I can check if I have it)

I can't find an achievement like that, but there is the "I Can't Eat This?" achievement for having the Evil Pie Plushie in your collection (along with other pie plushies).

Near as I can find there's one and only one Achievement for buying from the Crystal Shop, and you can buy any item from it to get that one, and you only need to buy one item. It just so happens that many items that are rare, totally extinct, and/or hard-to-find are often found in the Crystal Shop. Of all these items some have become that way because users have been taking them out of circulation to complete an Achievement for their own.

The only "buy x from the crystal shop" achievement I think we have is for

Oho! is right and I am wrong there is one specific item to buy from there. It's not in the shop now.

That must be it! I probably remembered it wrong, Thank you very much <3

Thank you for your help ^^