I've found that if you walk in to any fabric store and ask for ripstop nylon, you get mostly blank stares.

However, every store I've been in had some. Look under "rain wear" or "outter wear" or simular sections. Tell the employee you are looking for raincoat, tent, or decretive flag material.

I've found the stuff in Hancock, JoAnne, and G Street fabric stores in this area. Different colors and wildly varying prices at each place, so it pays to shop around.