Do you mean backing tracks to actual songs, or just random progressions for you to jam over?

There are many sites out there but I find YouTube to be a pretty good place to look. The sound quality may not be perfect but there is a lot out there and searching is quick and easy.

Check this one out for example: Blues in E Backing Track

If you want to make your own backing tracks without having to spend a lot of time recording, check out Band-in-a-box. Great software for making quick backing tracks!