I went through this, for one cap from the same people, a month ago. I had hoped they'd be like the Fantastic Fan people and fall all over themselves to take care of their customers. No such luck!
I finally pried the trick out of them though: You have to call their West Coast Distributor, Rob Soares at 602-821-7477. I explained to him that I only needed one cap to replace a broken one I had and he told me he'd call the factory (Debbie and Alan) and authorize it. There's apparently a concern at the factory that sending out one (or two) caps violates their agreements with their distributors.
A single Bayonet Plug (cap) costs $9.40, without the float (since I can re-use the old float).
They also told me that these can be bought from:
By the way, the www.batteryfillingsystems.com
website tells us that the Interstate number u2400 battery uses their float #34