Catering Wait Staff
If you are working with a reputable caterer, they can supply or
make recommendations for serving staff. If you are not planning on
using a caterer or restaurant, consider the teenage children of
friends and family - people who may not be close to the honoree or
otherwise attending the reception.
Your city might have a temp agency focused on catering. Or
perhaps a local university has a student employment agency with
trained catering and bartending staff.