After trying to buy a 128x128 body texture pack, a message popped up saying that there wasn't enough in stock. Is there stock for the in-game Torishop items? and if so, when does the stock refresh? Or is it because I'm using Shiai tokens?

Hey Maxiboo,

There is indeed stocks set both for in-game and forum shop. The stock refreshes periodically, and those times differ from item to item and there's no set date when it refreshes for all items. Some items stay out of stock for a very long time after being bought.
I suggest you try and utilize the forum shop to see the exact stocks up to date.

Restocked 128x128 packs, you should be able to buy it now