I need to find a reliable source for old tractor parts. I'm looking for a part that I don't know the name so it would be hard to do a search not knowing what it is called.
I have an old Ford 800 tractor I just purchased. [Hurray, I finally got a tractor.

Not much to look at, but I always knew that "There's no telling what I could do if I had me a tractor."]. It also came with an old shredder and a drive shaft to (almost) connect the two.
The tractor has a 6 spline PTO shaft. Some of you may know that there are two sizes of shafts, this is the larger shaft (I think 1-3/8"??). Usually a shreader will have a smooth shaft with a shear pin that will break if you hit something solid to protect the gear box. However, the gear box on this shreader has a 6 spline shaft just like the tractor. Unfortunately, the drive shaft I have has the yoke for the smooth shaft. I am looking for the part that connects the drive shaft to the shredder. It would have six splines on the inside to slip over the splined gearbox shaft and smooth on the outside for the drive shaft with a hole for the shear pin (grade 2 bolt).
What is this thing called or where should I call to look for one? Most tractor houses here sell new parts and specialize in certain brands (Deere, or Massey, or Case, etc.) and sell new parts. I am looking for parts for old equipment and would rather buy a low cost used part rather than the high-dollar part for large-scale farmers. Yes, I did try Tractor Supply and looked through their parts and did not find what I needed. They did have a drive shaft with the splines on both ends, but that was for tillers, etc. and I need something with a shear pin that would break away in case I hit something like a stump or old pipe, etc.
Thanks,
Dale (DH of Mary, TX)