But how do you find the seeds you need? There are many markets available for selling just what you need. On the other hand why not just save part of one years crops for the next years garden? I can't think of a single crop that doesn't have it's own built in seeds. Some we eat (ie Green Beans and Tomatoes), most we clean away and discard (ie: Peppers and Cantaloupe) which is a shame. Some even grow on the vine (ie: Spinach) I leave a portion of everything from my garden for a new years worth of growth. It's simple to do and is a sustainable way to keep the crops growing.
Removing seeds for preservation works in several ways depending on the produce. For those crops with edible seeds, for example okra and green beans, I usually leave one or two set aside and let them dry out. Once they are dried I open them down the middle and remove the seeds. For those plants where the seeds are removed before eating, like peppers and cantaloupe save some seeds and set them aside to dry out. For root plants just let the roots grow (see Plants That Reproduce With Little Or No Cost) and plant them in the ground.
Maybe this year you want to try growing something new, like a cucumber for example. You have two options for getting seeds. Buy a package of seeds from a lawn and garden store. This can be somewhat of a gamble. You'll never know what chemicals these seeds have been treated with. On the other hand you probably wanted to buy a cucumber anyway so why not buy an ORGANIC one and spoon some of the seeds out of the inside. You won't need many. Maybe five or six. Set them aside and let them dry out before planting.
Gardening has been the most sustainable human resource from the beginning. Once you get started like most you'll probably get hooked. The easiest and most inexpensive way to keep the planting going is to save your own seeds for planting. I gaurantee you won't be sorry you did.