This will be a short diary, and if because of that it violates Daily Kos guidelines, I pre-emptively apologize. I don't have alot to offer on the subject, but this might help a few people.
Vegetable seeds can be bought at many retail locations, many of which you are familiar with. Home Depot, Lowes, and Walmart (yeh, I don't like them either) all have large selections. Home Depot and Lowes have some selection of berry bushes (strawberry, raspberry, blackberry, blueberry and sometimes grapes). Additionally, Target and Dollar Tree both have limited selections.
Most packets have 30-100 seeds, and cost anywhere from 90¢ on up to about $1.50. Dollar Tree has theirs for 11¢ each.
Additionally, online resources have a much better selection.
Vegetables, herbs, fruit trees:
Gurney's Nursery
Burpee's Seeds
Garden City Seeds (These guys have the pepperoncini seeds, like you get with Papa John's pizza)
Specialty plants:
Raintree Nursery (Olive trees)
Montoso Gardens (Chocolate/cocoa, coffee, cinnamon.)
Freshops (Hops, for beer.)
Midwest Supplies (Hops.)
Seedman (Tobacco, curry, many exotics)
GaiaYoga (Coconuts, bananas)
Worldwide Plants (Bananas, email for prices.)
Baobabs (Vanilla, european source)
Seeds Hawaii (Vanilla, american source)
Reimer Seeds (Peppercorn/black pepper, baby corn for stir frying)
Millington Seed (Sugar beets. We looked it up, roughly 64 plants to make a pound of sugar.)
Howe Seeds (Oats, barley, wheat, flax, soy.)
Some of these are exotic, and would require much care and attention, perhaps even an expensive heated greenhouse. Then again, if you worry that there won't be much coffee shipped here at some point, do you really want to be rooting around in weeds, brewing them and pretending it's almost as good as the real thing?
Others yet are really only plausible for someone that owns a small farm already, and even then in theory (processing sugar beets to make granulated sugar might not be something someone can do on their own). But I included them because I was curious, and it might spur someone else to do something.
I've already started with cocoa myself (from Montoso), and most or all of the seeds are germinating. I'd have gotten coffee and cinnamon from them as well, but these are seasonal and won't be available until the fall.
Small fruit trees are also available at your local nursery, small enough to fit in a 5 gallon bucket on a patio or back porch, or even inside if you live in an apartment like many of us.
Others are better gardeners than I, and know more about it. I'm just starting myself, after all. But maybe if I put it here, you'll click through the links and order something for yourself. It's just a few bucks.