The places which remanufacture them need a core to rebuild. They sell a remanufactured unit from the cores. I'm suspicious that these places on EBay might sell used valve bodies or rebuilt valve bodies from junkyard transmissions. Check this place on
WWW.ATRA.Com. I'm not saying that they are junk, but just be cautious.
A few years ago there was a shop selling extremely cheap solenoid blocks, solenoids, and other parts online. They might have been in Florida. Several members from this site purchased from them, and had transmission codes. The parts came in bags which were saturated in old, smelly transmission fluid. The place said that they always coat the parts in transmission fluid to prevent rust during storage & shipping. They turned out to be junk parts which came out of junkyard transmissions which had contaminated fluid. The place had numerous complaints with the BBB, and the guy stopped returning emails. They even went one step further to pocket money from members on this site, and never ship the merchandise. I can't remember the name of the shop but it's not important since I doubt that they are still in business.