Seed barley is usually available at a Farmers Coop Elevator or a seed dealer. As far as I know the minimum quantity would be 1 bushel and would plant about 1/2 acre. You might be able to get smaller quantities from an organic coop but at a higher price.
It depends on how much you are buying. I see that the barley (Conlon 2 row malting barley) is about $5 a pound and there is about 58 pounds per bushel. I would expect to pay about $12 to maybe as much as $18 per bushel for premium seed quality barley at my local Coop elevator. If I were planting a small garden plot the 1 pound package might be fine but if I'm planting 50 acres (1 1/2 bushels per acre) it would be way beyond my means for recovering my costs.
I raise wheat and barley in Montana.
Used to sell 2-row (merit) on this forum, but they shut me down as I wouldn't buy a vendor membership that would have cost me more then I made. In fact if I offer to sell you some here I am breaking the rules (can you tell I am a bit upset with stupid rules). Piss on it... if you want some I will send you whatever you need for shipping plus a few beers. fifelee at gmail
Years if not decades. We have used seed a few years old and there is no "package" on the farm. It is all stored in large grain bins. I have never tried, but I bet 10 year old barley would still germinate over 80%.