While my landlord claimed that spare parts were not available, AppliancePartsPros made it easy. I just used my microwave model number to find the right part and it showed up quickly. Installation couldn't be simpler either--simply remove the glass platter, pull out the wrong ring, drop in the new one, and put the platter back on.
While I was at it, I also replaced the burned-out light bulb that sits under the microwave and over my stove. That was a little harder, needing a screwdriver to open the cover, but took only a few minutes.