So I have Kenmore Elite HE3 washer, 110.45872400, and the rear drum support corroded and broke like so many others. It unbolts from the inner tub but I cannot find the piece available separately, I see them available for whirlpool washers who I understand provided the parts. Can anyone provide me a way to cross reference so I can locate this part?

I also had the spider fail in my Kenmore front load washer. I tried looking around everywhere to get a new spider and no luck. The PN on the spider itself is 131450000. I phoned Sears (Canada) and asked about a warranty and they said they don't have Lifetime warranties on anything except hand tools so I was SOL. A new stainless drum and rear plastic drum would be $366, and just the rear drum with seal and bearings would be $255. They didn't sell just the stainless drum/spider.
So - I made my own spider with some 1.5" steel channel, a bit of round pipe and some angle iron. The steel shaft does break out of the aluminium spider assembly and fit nicely into my round pipe. I put a bit of electrical tape around the shaft for a nice tight fit into the steel pipe which I cut about 1.5" long. I cut 3/4" angle iron into 3" pieces and drilled and tapped for M8 bolts (metric) for the OEM bolts which hold the existing spider to thread into. I bolted the 3 of these into the drum and then also bolted these to the 1 1/2" channels (about 12" long). The other end of these channels got welded to the round pipe holding the shaft which was also welded into the pipe. I painted it and all works awesome. I took great care to make sure it was balanced and needed to put 2 washers under one of the channel arms where it was bolted to the angle iron.
After building and assembling it, I realized why the spider is not a separate part available to purchase (other than trying to get you to buy a new washer) - the reason I think is that when the stainless drum is bolted into the OEM spider, the bolts stretch the stainless hole into the spider so removal without breaking it would be almost impossible - and so installing a new spider would also be very diffficult if not impossible because of the stainless drum material being pressed into the spider when the bolts get tightened. (Kind of dificult to describe but it you remove a bolt you will see the deformation of the stainless which kind of protrudes into the spider.)
Anyway - I'm happy so far with the new steel spider. I also installed a new seal and the 2 drum bearings. For my washer, the Seal is a 40x80x10 and the bearings are 6307 2RS and 6303 3RS. Pretty cheap on ******** or Aliexpress. My washer is a Canadian model PN 970-C41072-00 which I believe crosses to a 417-41052-00 but it appears that many use this drum assembly. Have a look on Youtube on dissassembly which isn't too difficult if you want to give it a try. What have you got to lose - the washer is cloe to NFG if you are reading this. If you have access to a welder and a grinder building your own isn't hard either - just take care to balance it so it rotates without wobbling although I really wonder how critical this is because many times the laundry makes the tub off balance too. Sorry - no pictures - I can't find anywhere to attach. Have a look on Youtube to see what others have built - that is where I got the confidence to build my own. Good luck.