There is a Chamberlin's store not too far from WDW. If you exit WDW property from the Downtown Disney hotel area as if you were going to Goodings, then turn left instead of going straight across into the Crossroads. Go straight on this road (535 turns off....don't stay on 535, keep going straight) Eventually you will arrive at Sand Lake Rd. (also accessible from I4) Off of Sand Lake Rd at the intersection with Dr. Phillips Blvd is a large shopping center with an Albertsons. In this shopping center is Chamberlins.
Alternatively you could take I4 to Sand Lake Rd and head away from International Drive.
Its a smallish store, but you can probably get the basics here. I believe there is even a little in store cafe.
It's much closer than Whole Foods (which is a beautiful store...but we stop on the way down or not at all....)
You might want to check some of the grocery stores too.... I know certain chains in my area carry soy products, but not really the wheat free stuff.