Quigley must (
) use standard parts for the bushings, rod ends, springs, bolts, etc, I mean these are the same people who take the front end and T-case from the F-series instead of doing all custom. If you can get a part number for the bushing maybe you can trace back to the original application and find a poly for that. Surely they don't custom make the bushings?
Otherwise measure and call a 4WheelParts, as switchfoot already said, they stock a lot of poly sizes AFIK. My bet is they're from a Ford.