(for those in the U.S.) has some nice cotton-blend yarns (their Comfy yarns are made of cotton and acrylic and are nice for summer wear); you also might look at some of the bamboo blend and linen yarns. There are all kinds of different yarn fibers out there, so don't just stop at the most common ones (I've seen sugar cane yarn and even paper yarn).
Do you have a local yarn store that carries high-end yarns? That might be a good place to look.
Once you find a fiber you like, then you can look for it on sale somewhere.