They come up occasionally on eBay but can be quite expensive.
I paid something like a tenner for mine, but it came locked onto the filler pipe and I had to get a locksmith to make some keys to fit.
It still worked out a fairly cheap purchase though.
they come up on here from time to time, swap meets theres always one or two quite often without the key so you need to source a key try pming tubdub or woodslat see if they can keep there eyes open in the states there reliable and damned good with their pricing and shipping...