While my rack was in storage, I lost the screws to secure the factory crossbars onto the roof, but have been unsuccessful in obtaining just the screws. Every place I've contacted says they can order the entire crossbar set, but that's quite expensive.
Do they have a special head or something? If not, I'm going to guess they're an M6 thread pitch, which you should be able to buy at any hardware store. They're so cheap, you'll spend more on gas driving to check them out and see if they'll work.