Craigslist is good to browse once in a while too. Last year I found a lady who was selling a mint 15lb Ebonite V2 that her son had bought and used maybe 20 games for rec bowling in college. She sold it to me, with a bag, for $20. I traded it for a NIB 15lb Lucid that has earned me a lot of money.
I also love checking out the balls on the house racks when I bowl tournaments. I've never taken a ball from there, but it's neat to see what's lying around. The last center I was at had a ton of prominent balls. There was a single drill OG Black Widow, a Track 920T, an Ebonite Cyclone, and a Lane 1 Super Carbide Bomb. All of them were in very nice condition too.