looking for knowledge/parts for aftermarket saddle tanks

[YellowMenace]

My truck has a pair of ,what appear to be, aftermarket poly saddle tanks. The tanks look like they're in good shape but need to be cleaned out. The problem is that the cap for the right tank has been cracked (apparently by the tire when the bad springs and shocks were still on the truck). This cap is a female fitting that goes over the filler neck with coarse threads. Anyone familiar with a tank like this? Anyone know a source for a new cap?

The tanks seem to be made for this truck and not universal so I'm hoping someone else has them. I haven't yet removed one of them to look for markings.