Many online retailers sell moisture-wicking headwraps -- Performance, CRC, et al. Prices and styles may vary.

Besides wearing mine (called a "Buff" from CRC) for riding, I wear it when I'm working on the bikes; for some reason, even though it's not physically intense, my concentration on what I'm doing makes me drip sweat. The Buff stops that. Sucks it right up, and evaps at warp speed.