Take a look at these, just about any size you may want.
http://www.coyote-gear.com/4inch.html
For $99 with a vented cap and petcock bung, that is not a bad price. If I was looking for just one for a behind-the-seat tank, those would fit the build nicely!
I just got my Concorde XKS moped tank, which looks like it might work out ok with a little work (and a lot of paint).
