I have done the exact same thing as you are, only to a lesser extent. Last year I planted crimson clover from Peaceful Valley with excellent results. Not only do I use the crimson for fixing nitrogen but for recruiting bees to my backyard. I planted the seeds last fall. They grew to about a foot +, flowered in the spring and a lot of bees showed up, sticking around for the gardens I planted later.
This year I am trying buckwheat as I have read earthworms are very attracted to this plant and good for composting.
Bountiful Gardens was the place to go for this. They will ship small quantities for reasonable prices.
I am also going to try inoculated alfala- from Peaceful Valley, and regular from Bountiful Gardens. I like the alfalfa grown in the fields around my area and am just experimenting as which grows well in the backyard. Good luck.

If you are in zone 5 you might want to try winter rye because it is hardy to cold temps. Also try your closest farm feed fertilizer store, they may be able to order some for you and save you shipping costs.

How long any seed stays viable depends on how it is stored. Most all seeds need a cool, dry environment, someplace that is around 45 degrees and fairly dry. To get the dry atmosphere you could use a dessicant to pack with the seeds, and those shipped long distances may well have a packet included.
A local seed store may well special order something for you, seeds that are not normally grown around you, but you will still pay the shipping cost although they may be less than what you would pay. Most of use do not see shipping costs since normally that is included in the price we pay, not sperated out as the mail order houses do.