Here's a list of the stores at Dulles that sell SIM cards. Note that some of them are post-security in the Concourses and some of pre-security in the Terminal building.
Airport Wireless
Faber News
Forbes (only in the Main Terminal) The cards are on order.
Fox News Channel
Hudson News
News Connection
Stellar News B46 News (not bookstore) & Main Terminal East (ticketing level only)
Techshowcase
MacTrade (only international, not USA cards)
Washingtonian (only in the Main Terminal) – The cards are on order.
Here's a map to all the shops at Dulles:
http://www.mwaa.com/file/DullesSDMap.pdf