Where can I see all available Playstation 3 avatars?

There are several available on the PlayStation Store. Most of them are not free ($ 0.49), but it is possible to find free. You have only to run on the PlayStation Store, select Themes and avatars, and then selects Avatars. The ranking is depending on the game.

If you are a PSN Plus subscriber, there are a ton of free ones available. Otherwise you can get them from the PSN Store. I believe there are some free ones in there as well but, most cost a little bit.