In the midst of restoring my grandfathers '48 I've replaced almost every wear item on the rear suspension (it's a well-used farm truck). During the final going over I noticed the Rear Spring Seat section in the shop manual http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0461.htm

My bushings are starting to break up with age but it doesn't seem like anyone sells replacements. Does anyone know where to buy new rubber bushings or if there's a substitute part lurking out there?