most good corsetiers will do custom, but you're better off if you can get it done by someone who can do a fitting -- or is willing to ship back and forth until you get a corset that fits properly.

I make corsets myself, but if you want a recommendation, check out Xcentricities at www.corset.net. They are friends of mine, but they do great corsets. Expensive, but worth it. For custom work they are willing to re-do corsets if the first one doesn't work, without charging for the second, or 3rd or whatever it takes to get you the corset you want. They also sell ready-made.