If you want to buy an STi Mobile phone, or refills for one, or refills for just about every long distance and cellular calling card out there, CheapPhoneCards has what you need. Plus you get points for buying stuff and referring others to buy stuff so you can free phone cards and even phones.
P.S. You can now get an Oxygen prepaid (Cingular network) phone from CheapPhoneCards. If Cingular is much better than Sprint in your area and you can stand the phone or get another GSM phone this is an OK option, now that they have $10\30-day cards that are marked down 15% by CheapPhoneCards.