Go to a mattress store that sells waterbeds, they should have some extra caps. I have also found waterbed fill kits a hardware stores with a filler nozzle and a set of extra caps.
or just use drain plugs that cost about $.75 at your local hardware store. They have a metal pull ring, are more tapered than standard fat sac plugs and no need for screw on caps. Get a dozen and never be without again.