Plan in advance: buy your garden seeds discounted in September and October for the following spring. I got almost all my seeds that way; I spent about $10 for $100 worth of seeds, esp. corn seed which is more expensive than others. If you keep them over winter in a cool dry place or a jar in the refrigerator they should germinate fine.
Also try to purchase heirloom or non-hybrid seed so you can save seeds from the plants from year to year. We saved melons, pepper, squashes, and Indian corn seed for planting this spring. We also have our own seed potatoes from those leftover shriveled potatoes from last year that have begun growing eyes down in the basement.
You can also exchange with friends and neighbors. Usually our church as well as my local Curves gym has a plant exchange in the spring: you bring some of your extras and get something different.