There is at least one company that offers do-it-yourself kits to create the mold. Everything else is through mail. Best Buy could sell the kits and then you would be able to order them from the company based on what you purchased from Best Buy.
Thanks for the suggestion, jayson! Creating custom in-ear monitors is a very specialized service but certainly one that there is a demand for. We are constantly watching for new product offerings and partnerships so we appreciate the feedback. Keep an eye out for new product offerings in our ads and BestBuy.com Musical Instruments!