It's part of the fuel pump hangar assembly, and as far as I am aware it is not available new. I don't think it can be had for a similar mazda platform, ie a 323 or mercury tracer.
Used may be your best bet, if you can locate one in the junkyard.
I do recall seeing a 323 GTX guy having similar issues, unable to secure a replacement, he rebuilt his. The mechanism is a wire-wound potentiometer, the wire had worn thru where the wiper makes contact. He measured the wire gauge, found exact wire replacement and re-wound it to bring it back into operating order.
I think that was on the clubprotege forums. You might still be able to find it there, or I may have posted a link on teamcapri here in the past. Who knows with the photobucket debacle if the pictures are still available