Per Danny D.:
To my knowledge, NOBODY sells just the bung cap, at least,
not for the purpose of sealing a portable gasoline container.

aMuzi had my hopes up with
http://ezpourspout.com/ez-pour-produ...ase-cap-black/, but
the description seems to say it won't fit most cans:
"Black cap is fine thread and typically fits older Briggs and Stratton,
Rubbermaid, essence, and Gott cans."

i.e. there's nothing even remotely "fine" about the thread on my
jammed-up Scepter can.
--
Pete Cresswell