My advice would be to go to bicycle shop and check out the selections and the prices. They should also be able to provide technical assistance. Another possibility would be to find a catalog from a company that specializes in bicycle parts and equipment and compare.
I would ask one of your dirt bike buddies. If they can not help you then I would search the web for good places. Some towns have dirtbike/motorcycle junk yards or try a swap meet.
Dealership's but they are priced high.You can also buy them used on ebay or if you have a local dirtbike/motorcycle junk yard close to you can get it real cheap.