Basically, here's how you go about finding a suit.
1) Find a suit. Department store, store in the mall, Men's Wearhouse, whatever. They all have different styles, so keep in mind what you want. Just realize that the more modern, slimmer styles you see in more expensive clothing stores are more expensive.
2) Make sure the shoulders fit, and I guess, that the coat is the right length for you (short, med, tall). Turn around and make sure there aren't any weird wrinkles. Make sure the waist on the pants fit, obviously. The leg length can be shortened by a tailor.
3) Take it to a tailor. So long as the shoulders fit and the overall length is good, they can do things like waist suppression and shorten sleeve length readily.
Note that despite what people say, this isn't necessary (although it might make a slightly better impression??). I didn't really have the cash to get another suit/tailoring and went to all of my interviews wearing a black suit that was 1 size too big and not tailored (+ ludicrously cheap off-white shirt, blue tie). After all, I'm a student, not a model (there's a ridiculously long thread about this in the PreAllo forum which involves one guy sending everybody into hysteria as he talks about buying dress clothing far out of the reach of the average pre-med. Good for him that he's so knowledgeable about clothing, but I don't think it matters that much). Anyway, I got into medical school, frumpy suit and all.
I guess I'll buy a proper suit when interviews for residency come up.