If you know of any better sources, please post!
Eros Entertainment is the largest distributor of Bollywood movies to the rest of the world and the films you buy from them will always have the highest quality (sound, picture, and possibly translations). They have made their website much easier to navigate in the past year.
eindiastore on Amazon is a reliable seller. I have personally bought several things from them, both music and movies. They include a free DVD of something you don’t want (you can request to not have this); the extra thing is sent sans case.
Yash Raj Films stopped selling online, which is unfortunate, because I would definitely invest in some of their films.
Rajshri Films does not sell online, but you can watch their movies on their website for free (without subtitles).
The Indian store in your city most likely sells current Bollywood movies and can get older movies for you if you ask.
I do not know if regular library suppliers, such as Baker & Taylor, sell Bollywood products. To be honest, I have had the most luck with Amazon if Amazon is “claiming” it.