You could also add WKYTP, (West Kentucky Tractor Parts), a salvage yard like Southern Global, to that list.
WKYTP did not have any. I know, I live just across the state line from them. When my JD850 head blew, nobody anywhere had one .... except Fredricks. I was recetly at Fredricks a few weeks ago. They have both heads and complete engines. Well, when my mechanic opened up the 3T80 engine, there was more damage below. It would take $4,000 in parts to rebuild the 3T80 on the JD850. Then with labor the price tag got closer to $5K. The cost of a totally rebuild+refurbish Yanmar YM2610 with the JD850 trade-in was a better deal at $5,700. My JD850 also had white foam spewing from the hydralic lines too. The previous owner was very neglectful.
With all that said, if your YM2610 is in good shape for the rest of the tractor, then seeing how much a crated restored 3T80 engine from Fredricks could be a great option. FYI, Coldwater tractor parts mentioned they too have complete 3T80 and 3T90 engines.
On another tractor forum, a person is doing just this, an engine swap. It's not really hard since it's just bolts, flywheel gaskets, reconnecting the hoses for cooling and the throttle linkage.
I had searched high and low for 4-months for a 3T80 head. Either $2,200 from Deere (without springs and valves) or Fredricks. The YM2500 and another Yanmar use the 3T80 engine too.
