Earlier post discussed modifying or building a roller for a compost turner project.

Since the rollers I have will be great spares for the other 5209 in service, I will build a new rotor out of sch40 pipe.

Building sawmill equipment and many conveyor rolls, I'm ok with building a 7' long rotor.
I'm looking for a suitable spline to fit the existing drive system. The yoke has a female spline of 1.4" o.d x 20 teeth. I know the original shaft has a flat that locates the timing aspect of the roller grooves. In this case, not necessary.

Can anyone point me to a salvage yard that might sell me the end of a busted roller (they do break), or a good drivetrain house that might be able to match this?

According to my information the shaft on the end of the roller is a 1-3/8" 21-spline per ASAE S203.13 which is the same as a standard 1000 rpm PTO shaft. The spline in the telescoping portion of the driveshaft is a 20 spline but I don't know to what standard. Since nothing is intended to interface with the telescoping splines except for the parts of the driveshaft itself it may be hard to match up to using off-the-shelf, industry-
standard components.

As for your rotor I'd suggest using structural tube instead of pipe unless you're planning on running it fairly slowly. Tubing has better tolerances, is straighter, and will have better mechanical properties compared to pipe. The cost will be higher but there's a reason for this. The rollers you're replacing go through a fairly rigorous manufacturing process to make sure they are straight and balanced to prevent things from getting wild when running at the rotational speed that they do.