I went to a game at a convention and the guy who was throwing the game didn't have enough shield tokens. So I mingled mine in and lost count of how many. I am now short four shield tokens. Where can I get more?

I would think that any kind of token would work, such as glass beads or coins. However, if you want to get fancy, there are companies out there that are making clear acrylic tokens for X-Wing....

http://www.litko.net/categories/Shop-By-Compatible-Game-System/Star-Wars-X-Wing/

http://www.appliedperspective.com/acrylic/product-category/gaming/x-wing/

After you buy a few more ships you'll be fine.

Unless you're the type of person who wants to carry every single token with you everywhere, you'll find yourself with more of them than you need. I don't even bother to punch out most of the tokens that come w/my ships anymore.