The MS420 always did come with a rack mount rail kit for standard 19" 1U racks, included by default with the switch. Unfortunately I do not believe there was a separate SKU to order spare rack mount kits. And they might be hard to come by now as MS420 is End-of-Sale. Worst case, might have to buy a cheap rack-mount shelf.
I was able to call Meraki and get a replacement as long as I had a valid license. Have you tried calling Meraki 4159376671? What did they say? They may have SKU to purchase the proper model now as well.