I don't know if they do any clothing but Veach's in Ankeny has done some top notch work for me before on boots, saddles, bridles, etc. They're always friendly and I wouldn't hesitate to have them work on something if they say they can do it right. I need to make a trip over there soon.
A couple of years ago he did complete alterations on a pair of tall riding boots (in a week!) and they are perfect. I was so nervous about having someone cut into a brand new pair of $250 boots, but when he was done they were gorgeous and fit like custom boots which I'd never be able to afford. Too bad I pretty much quit showing horses and never wear them! Durrrr.