You can request them from other players. I see shops mostly in Re-tail. Or you can make your own! Or be lazy like me and just type some junk and slap a gif on it!
If you're referring to how to put them into your profile/posts, just go to where you change your avatar and go to the signature change instead. Also, signatures are an expression of yourself, so don't feel like you're confined to have some fantastically amazing signatures. Not everyone is that talented (especially me ;_ when it comes to graphics. However, there are some pretty amazing graphic artists on here that would happily help you out by making you one either for free or for Forum Bells. Or you could do what I do and throw gif sets into my signature!