Sites for buying horseshoes and horseshoe pitching accessories:

 I've bought from Ron and Polly, Dennis Ohms (Omega), White Distributors, and Lucky Shoe Pro so I can recommend them.

Ron and Polly's Horseshoe Pitching Supplies (formerly Harris)
White Distributors
Omega Horseshoes
Lucky Shoe Pro Shop

Independent Horseshoe Distributors Association
NHPA Sales Distributors
Mike's Horseshoes (Mike Littell)
Horseshoes and Horseshoe Related Items (Herb and Judie Heesch)
Kimmy's Horseshoe Shop
Pork's Shoe Shack

Manufacturers:
Cal-Flip Inc
Gordon Horseshoe Company
M & M Horseshoes
St. Pierre Manufacturing Company
Thoroughbred Pitching Shoes