I used to own a vintage trials bike. In that circle, Delkevic had a good reputation but the only place to get them was Europe. Well, I just saw this on eBay. The seller is delkevic_us.
http://pages.ebay.com/motors/link/?nav=item.view&id=172002195078&alt=web
http://pages.ebay.com/motors/link/?nav=item.view&id=172002195078&alt=web