Best to find a tranny shop. The best ones are the ones with piles of torn down trannies everywhere. They sometimes sell parts really cheap....if you can find them!
Junkyard would be tough because you would have to tear apart the valve bodies to make sure you have an exact match. Some yards don't want you tearing apart the trannies. There were numerous changes to the valves and valve bodies over the years.
I don't know if a dealer would even be able to get that part.
Another option is to junk it and buy a full manual reverse valve body!
BTW: be careful pulling those valves out.