"Pinetree Seeds" in Maine is the best source I' ve found for a really good list of open pollinated seeds at very affordable prices. Most packets are from $1.25-$2.00. I agree with Julie...buy more than what you need for your garden, plant what you'll use, and keep the rest frozen. They'll keep for years. It is much less risky experimenting with saving seeds when you have a reliable supply put away.

R. H. Shumway seeds also has nearly every seed you can think of, including a nice amount of heritage seeds, but they are a a bit more pricey than Pinetree.