Go with scrubs. They're easily available, inexpensive, come in a wide variety of colors, and if you hunt around, you can probably find them in your size.
Check out an industrial supply store, a store that sells only scrubs or even at a big box store like Wal-Mart or Target. I'd be willing to be that if they don't have them in, they can probably order the size you need.
As long as the material matches in color and fabric (to give the illusion of a one-piece flightsuit), you'll be fine.
