The cheapest providers are www.tele2.no and www.chess.no (the price is about 1 NOK/min to Norwegian numbers and 8 NOK/min to the US). I think you have to order the SIM online from these providers. www.telenor.no and www.netcom.no are the two biggest providers. They are more expensive, but I think you will be able to buy their SIM from local mobile phone retailers. I would recommend Chess or Telenor. I know that one can buy Telenor refill cards at most kiosks/newsstands like Narvesen and 7-11. I’m not sure about Chess, you might have to refill minutes online or by phone.