what for? The only time i've ever seen a capped USB cable is with an apple product, and its sort of functionally useless. Unless you live in a constant sandstorm or something.
Honestly, you'll likely not need one. I've got an old 64MB Dell USB drive that I've had on my keychain for like a year now with no cap. Every now and then I may blast the connector out with some compressed air, but it's never given me any problems. It's not really all that prone to damage, just so long as it can get a good connection to the receiving port, it'll be fine.
