Amazon! Especially with Amazon Prime for free shipping/returns, I've bought nearly all my bras from there. I use the UK sizing so I'm a 32FF, a size not carried in most of my local stores. By shopping on amazon I can get most of them for under $30. Heck I just ordered this one yesterday for $21.

You do have to know which brands/styles work for you and to get the cheaper ones, they may not be your ideal color. But what I do is check them every week or two and occasionally a basic color (black, nude, etc) will go on sale cheap. I just search by my bra size and style I like and sift through (so like I put in 32FF plunge bra and get tons of results). Highly recommended.