If you're looking for any (non-touristy) Bhutanese Buddhist items, this is where you probably want to shop. The proprietor is very helpful and honest. The prices are very reasonable. If you want to bring home blessed prayer flags, you can get them here, and Lama Tshering Dorji will give you his contact information so you can reach him in order to determine an "auspicious" date to hang them.
NOTE: You may arrive at the storefront to find that it's closed. Don't despair: Just call the phone number on the door and he will promptly appear to open the shop for you!