Bacon Sails in Annapolis has a used sail inventory as a start for a search, might be other places online if you looked. Also craigslist has listings, there are some on our local listings for sails. I think you will find that the cost of a used one is high enough that you may not want to spend the money and just save it for a new own.
I patched my old, original main several times with sail tape and then sewed it to hold it down, that would get you thru a season. Sailrite.com has videos and how-tos, if you have a home machine.