How can I find out the owner of the buried tank at the house I just bought. There are several suppliers in the area and two of them said it wasn't theirs and they would not fill it

Contact your realtor. They should be able to tell you or they can contact the agent that handled the home for the previous owner and ask them who it belongs to. This should have been in your papers unless it was a reposession.

I own my propane tank. Not all tanks are owned by a supplier. Of course this doesn't help getting it filled, but wasn't getting it topped off part of the sale, with a receipt from that?