What I'd like to get my hands on at decent prices are .223 Remington, .357 Magnum and .40 S&W brass. Those are what I expect to use in the largest quantities in the foreseeable future.
GB,
Something I started doing not too long ago is buying my brass on gunbroker. I specifically look up once fired brass in whatever caliber I'm looking for. If you use the search type in "brass once 223" or whatever other caliber you want and you'll see all kinds of it come up.
At one time I got 1700 pieces of 223 winchester for a little over a hundred bucks. If you're ok with once fired brass for much less money, gunbroker is the way to go. I also recently got 500 30-30 once fired remington pieces for about 50 - 60 dollars. Did the same thing with 30-06 brass too for about the same amount.
I hate to give away my secret fishing hole, so to speak, but I'd like to save people their money too. It's not perfect, but it's another option that I don't think a lot of people are aware of.