Auctions, both near and virtual, can be a good source for marbles. You might even luck into finding some at a flea market or antiques shop. It's getting harder to find them "in the wild", as we say here. Ebay of course, but stick with sellers you learn about who have a good track record for selling authentically old marbles, if you want vintage. Best advice I was given was collect what you like and try to buy the best of that type. Most of us spent way too much starting out, myself included.