Marine supply stores generally have a fuel cap that you can mount in the bed on the bed wood, if you want to put it there.
I know it's a common place to put it, I just don't think it is good place to do it. If you have a tonneau cover or a load in the bed, it will make it difficult to fill. I've noticed that GM Venture or TransVan minivans have a self rimming gas cap that might be easy to mount in a fender. Just a thought.
