As crazy as this sounds, the pair I've been using for years is a pair that my sister got me in the goth section of a Hot Topic store when she was going through her "dark" stage (good God, I'm glad it's over :shock: ).
Started wearing them because I had early stages of Carpal Tunnel that gave indications that it was going to progress real fast, and someone casually commented that they helped.
They're black, light weight, hook through the thumb and leave my fingers free to type/knit/etc.
Try local teen stores and the such, and if all fails, grab a really long tube sock, that will go slightly over your elbow, cut the toe off and to make sure it won't slip down your arm, make a small cut for your thumb to go through.
Edit: There's a type of armwarmers sold in bicycle stores. But they have lycra in them and I don't know how breathable they would be.