I forget the size of the bung/thread but when I looked a couple years ago I couldn't find anything from Pingle or other online suppliers that would fit. Found one from a scooter(Puch?) that was the correct size but didn't order it.
I entered the size and "petcock" to find it. Hopefully things have changed since then and once someone posts the actual size you might have better luck searching.
Is yours leaking?