I've used round robin seed exchanges. There is one box that can have up to a hundred or more varieties of seeds. This gets sent to you via USPS, etc. You take what you can use and put something in that you'd like to share. When you're all finished, you seal up the box and send it to the address that is next on the round robin exchange list. You can acquire many different varieties of seed in this manner.

http://forums.gardenweb.com/forums/roundrobin/