The place I knew was at www.godzillatoy.com
. They had a store on the north side of Atlanta and I was based down there when they were open. Little place, but packed full of stuff and they had a lot of posters. Sadly, they appear to be out of business.
You might try emailing these guys to see if they carry them or can point you in the right direction:http://www.videodaikaiju.com/
If all else fails, I would watch Ebay. You can put in search terms and have the system notify you of results when new items are listed that match your search.