Sword covers
There are a number of ways to find or create a decent sword cover.
Find
Tube socks make absolutely horrid covers, they are heavy and tend to remove skin with drag shots. Nylon tights are commonly seen on ultra light weapons as they are extremely light, however the seventh edition rulebook effectively has outlawed them by requiring covers be durable and opaque to be legal, Nylons are neither. Don't attempt to "find" covers, build them your self.
Build
Cut a piece of broad cloth 3 inches longer than the blade of your weapon and 8 inches wide, sew the long sides of the cloth together to form a tube. Sew one end closed. Ta-Da, you have a cover. Some people will sew the open end's hem such that it can hold a draw string rather than rely on tape to hold it on.
Recent experiments have used "rip stop" fabric, as used in kite design. Light, strong, and non-porous, rip stop is said to reduce both weight and wind resistance.
--- Links
All manner of socks, some with Very Cool prints: http://www.sock-dreams.com
Kitebuilder.com's fabrics page: http://www.kitebuilder.com/catalog/index.php/cPath/24
Check out the Amtgard Resources page for more information.