This site always has a good selection of helmets
There can be some real good deals to be had on some of their helmets... but they seem to be sold out of a lot of the cheapies at the moment...
Hop's fullface seems pretty good as well...
hugh, go on wiggle.co.uk, and look at the helmets there. Bell drops and giro remedys are popular choices for downhill, as lots of people on asra and silverfish use them. also try 661's for some cheap good deals.
although wiggle is in the uk, they did free shipping on my bell drop, and it arrived from the uk in 4 days.