I found this when I moved the spare tire. Is that suppose to be there?

Normal but I believe a plug is normally in there.

yes it's supposed to be there . not sure of the purpose , you should have a rubber plug to cover it . good luck .

Does anyone know a site that sells one?

unfortunately i dont think anyone sells that one plug, i was missing mine too. PO are retarded most of the time and remove stuff and do not put them back. I took some silicone and just put some over the hole until i found a rubber stop that i modified from a hardware store to cover the hole.

Just put a little silicone over it if you are worried until you find something to plug it with