You MAY be able to special order it from one of the better butcher counters, like Fancy's in Osterville or Petersen's in Yarmouthport, but I've never seen it anywhere on Cape. I usually get mine mail-order from The Stockyards in Chicago. Pricey, but nice for that special occassion.
If you really want the best...go the shipped FRESH route with Lobel's of NY. Dry aged 6 weeks instead of the usual 3, plus they use only the top 2% of Prime cuts. Not cheap by any means but you won't find better.