Walmart Fair lakes
Walmart in Fair lakes is the only Walmart in the area that has any fly fishing equipment. It is in the back of the store in the fishing section. There is a decent selection of flies, and tippet/leader material. There are a few fly fishing combos. The selection of accessories changes occasionally. There used to be a few fly rod cases (for 9" 4pc combos) but now it is replaced by a few trout nets. The store carries line clippers and other accessories, as well as a few fly boxes. Although this selection is not as vast as any of the other fly stores in the area, it may be a good place to run in quick and pick up some extra tipped or flies for a trip if you are in the area. It is a good place for the budget minded fly enthusiast. If you are looking for rods alone (not combos) or waders, try Dick's Sporting Goods Down the street.
The address again is:

Walmart (Fair Lakes)
13059 Fair Lakes Ctr
Fairfax, VA 22030