Some of these sites require registration (which is free).
Before you download just make sure it's compatible with your BB and your OS. I've also used themes on my 8310 (188.8.131.52) that were designed for 4.2.2. The ones that I've heard cause very serious problems for 4.5 users are RIM's Today Plus and RIM's Surreal, though I'm sure there are more.
Also, I found that some 4.5 themes didn't work until I reinstalled all the BB Dimension themes.
Out of curiosity, what theme did you install that nuked you phone? Was it a Bottom Zen, L, or Today Plus theme? Or was it a standard Icon, Today, or Zen theme? I also have an 8320 with OS 4.5 installed and 4.2 themes seem to work fine for me, so I am just curious.