You can get the black gaskets at D&B for about a dollar.
Being in the military vehicle and collectable hobby, I have gathered about 50 various cans. There is a whole separate faction of the military vehicle group that concentrates on cans. There are a few different ways to tell if the can is military or civilian. I am always interested in a can, provided it is in relatively good condition. (Some cans are worth stupid big money!) Some can go for as little as a $5.00, and up to a hundred bucks. Certain dates and variations can get quite spendy, even when in lousy condition.
Four for $117 shipped isn't too bad, if they are real military cans, and not civilian. You can buy locally civilian cans for about $15-20 each.
The fuel doesn't care if it is being stored in a real or civilian can, if simply storing fuel. I think I have a few extra DD's (you military guys will know what those are) , if anyone needs one (and there are issued and civilian models of those as well).