Sites for buying horseshoes and horseshoe pitching accessories:

 I've bought from several of these businesses. Choose the ones that best meet your needs and value for cost.

Ron and Polly's Horseshoe Pitching Supplies (Horseshoes For You)
Dennis' Horseshoes
Horseshoes Online/White Distributors
Omega Horseshoes (Their 1.5# Cadet horseshoes are great for kids)
KHPA Dealer (Kentucky) (They list Cal-Flip shoes
Dick's Sporting Goods

Manufacturers:
M & M Horseshoes
Thoroughbred Pitching Shoes