2xist's recently released Soy collection features several styles of briefs and a t-shirt made with 95% soy-based fabric (the rest is Nylon).
The soy fabric has natural antimicrobial properties, is very breathable, provides natural UV protection, is moisture wicking and is touted as feeling "more luxurious than cashmere." Similar properties to clothing made with bamboo or high performance fabrics derived from corn and other materials.
MSRP start around $19. Retail and online locations available at 2xist.