E-bay.
Look for "power sellers" selling brand-new helmets. I was looking around last year and found full-face, brand-new M2000 Bieffe's for less than $100. Still haven't bought one, though, but the sweaty loaners are starting to get to me. I think this will be next on my list after tires for next year.
With used helmets, there's a risk that someone took a spill and caused internal damage, or left it in a car trunk, or cleaned it with the wrong stuff (weakening the shell). They may be perfectly honest in their listing, but not realize what they've done, or they could just be ripping you off. The point is, you just have to be more careful with this situation.
