We used to be able to get some stickers in the Cash Shop, but now I can't find a place to buy them? Am I just missing it or are they gone?

I believe all of the ones that used to be in the shop (at least under the 'stickers' category) have been retired for now, so you'll have to get them from other users or wait for some of them to be brought back during Black Friday or some other time when they do temporary returns of old items.

There are a few in the themed sets section, but that's all for right now.