For old blues 78's subscribe to VJM, the quarterly world wide jazz and blues auction magazine. Details on www.vjm.biz.
Just for the record (coarse groove of course!), 78's were still made and could be bought in Britain up till 1960. Nothing to do with not having as much money as the Yanks (both formats cost about the same anyway). They carried on making them mainly for export to third world countries where they had no electricity and still used wind-up machines.