Sources for used trophies:
Thrift shops, especially if you can get to the place where the pickup trucks take their stuff for sorting.
Little league, PAL, etc. It is common for the larger youth sports organizations to end up with excess trophies that they can't use anymore. I know when I was on the board for a PAL soccer league we threw away dozens of trophies every year.
The problem, of course, is that these trophies are likely to have a soccer or baseball player on them instead of a motorcycle, but you may find some generic trophies.
What you should also do is encourage the winners to skip their trophy. The kids are going to want one, especially their first one. But Mom and Dad will probably discard it as soon as they get it home so they would be just as happy to donate it back.
Rod