http://www.ps3-themes.com/
That's what I use anyway. I have it set as the homepage for the PS3 web browser so I have a quick peek whenever I turn the PS3 on to see if there's anything exciting.
The current trend (sadly) is towards dynamic themes that suck. Everyone puts a little postage stamp sized video playing a games trailer in a corner, or the other one is having the entire background be a really low quality GIF style animation.
Occasionally you see some great themes though.
Aniom, Aniom Taiga, and
Aniom Koira Evolution are all available at PS3 themes.
Sadly my favourite Aniom theme was actually Aniom Tesla but I can't see it at PS3-themes. I have it on my PS3 though so if you're after that one in particular then let me know and I'll see if there's a way to get it off my PS3.