It's part of the gas tank filler neck. I may be wrong but I don't think it's serviceable separately. IE it's part of the filler neck itself. If so, a replacement filler neck should fix you right up.
Note that I said "replacement" and not "new". I believe that Ford has deemed the filler neck for 98 and older MY's as obsolete. So the after market or salvage yards are your best bet.
One source for salvage yard parts is car-part.com.