You can have the OE one stripped, polished and re-sealed IF the pitting isn't deep into the metal. If it is, either go the route Jon B suggests or give Paul Scharf a call.....he may have a neat one off a wreck that would be less expensive than a new OEM replacement.
BTW, they are easy off without instruction, getting it back on involves a trick (there're a couple threads on that here, but they're old).... there's a hole in the filler tube (visible but not obvious) that can be used to hold the tube up (using a heavy-wire hook.....I actually use a barbeque skewer with a hook bent in one end) so you can reattach the cap around it. The descriptions in the older threads are way better, and there may be pix as well.