I think you can forget the retailer, as the percentage of people wanting this size will be so small that no retailer will risk the money to stock it.
The only possibility locally is the Sportsman's Guide off Hardman Ave. in S. St.Paul. In my view, it's a craptastic jumble sale, and yes you will spend a LOT of time looking through everything, but they have so much junk that you MIGHT get lucky.
Other than that, Google is your friend:
http://www.bigcamo.com/big-tall-hunting-bibs.php