electomagnetic clutch
I do not think the part is all that rare.
About a year ago, I pulled mine from the compressor and went to a couple of shops in an industrial park specializing in auto air conditioning. Second shop had several on the shelf. Suspect that most shops that do enough volume with Yorks would have that part. You may also be aware that Ford and other makers used York compressors, so even a Ford or other dealer may be worth a shot. Then, there's always the myriad of auto parts sellers on the net and if all else fails, the boneyard may be your safest bet.
Good luck.