Any suggestions or sources on what to use for replacing the rubber gasket on ammo cans?

Silicone?

the "bum" gaskets I have on my mil-ammo cans are way too much trouble to horse with.

I just take an oversize sheet of a good sheet plastic (you can also use plastic storage bags), and drape them over the open top; then seal 'em down. trim to suit.

I hunted that dog for a long time.  HEre is the result of my search.  The ammo cans are made to be disposable.  They are loaded once and kept in storage until used.  When the seal is popped, ammo goes away, can is trashed.  So, nobody makes replacement gaskets for them.  Which is sad, cause I have 20 year old cans in perfect shape that just need new, non-compressed gaskets.  I have learned to start putting silicone or , even better, 303  Aerospace Protectant on them , as well as my Peican case gaskets - it is great stuff.  Even if your ammo can seals are almost shot, that stuff may rejuvenate them.

It does seem odd though that the company that originally produces the seals for the cans is such a secret.  I have never been able to find them.