I think California Tints is one of the cheapest places. You can also check this for some local places.
http://www.e90post.com/forums/showthread.php?t=252189
As far as legal, 70% transmission is the legal limit for front windows in California, and I think factory windows are close to that already. So basically, there isn't any legal tint you can get for the front. As far as the windows in the backseat, you can go as dark as you want.