All else fails, BUNA-N sheet gasket and cut your own
Get something resistant to todays Gasoline + Ethanol and cut ones out, most likely BUNA-N but ask the counter man. They need to be about 1/2" wide to cover up the vent hole in the rim of a real US Military style can.
To make the canter hole you can get a piece of EMT or thinwall tubing the right diameter and sharpen it into a gasket punch. Couple of pieces of soft wood as a backing plate, a mallet and all done.
For a spout you want to use a narrow gasket or a big O-ring to keep the vent hole open - Note the hole at the top of the gas can spout seal flange. For that you need to order a small bag of 2-1/2" x 1/8 O-rings OR you have to notch a regular gasket at the air vent hole.
3/4 - 7/8 OD spouts that will fit inside a modern Unleaded tank restrictor flap are still available, but since they don't meet the current Spill Resistant regulations you'll have to find them at a Surplus House.