Saint Louis, MO
check out www.realself.com There are a lot of patient reviews in there - of all kinds of doctors, but the majority are plastic surgeons. Make a list of all the plastic surgeons in your area, and then go to their Web sites and see if you can tell if a majority of their work is doing body contouring for massive weight loss patients. And check all the reviews you can find, too. Then narrow it down to 3-5 surgeons and schedule consults with them.
I don't know of anyone off hand who specializes in it in St Louis (but then, I wasn't looking in that area), but make sure the person at minimum does a boatload of them. It's a complicated subspecialty, so you want someone who does a lot of these things, not just three or four times a year.