Adding to this in case others are in the same boat.
(Slight thread drift.... I wanted to purchase the upgraded power supply so we could plug a VOIP phone into the iMac and share the WiFi because the phone is in a spot without a hardwire option. This worked perfectly on my iMac so I hoped to get the upgraded power supply for the other one we bought.) /thread_drift
Stopped by the Apple store and the manager could not figure out how to order a power supply for the M1 iMac- He essentially started walking toward the accessories shelf and when I mentioned "M1 iMac", he stopped and said he didn't think those could be ordered. He told me to contact apple... which I explained I had already done multiple times and they told me to go to an Apple store.
He contacted someone "in the back". This person tried to look up the part and, after quite a bit of searching, was able to find them, but seemed confused as to how to order one for me unless it "was treated like a part I didn't receive with my initial order". Most were out of stock, but they had one color. Price was $109.
After talking with him, my plans changed a bit and I ended up with a Belkin USB-C to Ethernet adapter. Will hopefully solve my problem without the expense of the power supply.
(side note: Apple customer service was unable to find the Thunderbolt 3/4 adapter to Ethernet for me and, I assume, didn't know the USB-C adapter would work (which I didn't either)).