After just going through this same scenario (bought a receiver elsewhere), understand that Dexcom WILL NOT warranty the receiver for any reason, and WILL NOT sell you supplies, and will refer you back to your DR if you have any difficulties, even if you have a valid RX at Dexcom until you have been prescribed and issued a receiver. For those undergoing upgrades the process is very different. Because the receiver is an RX only item (burnished into the back of the receiver, so it's not an insurance issue), except for a few states it is illegal to transfer it. That said, I found that since I am buying supplies off the street, the G4 was the best model for me. The transmitters work longer 6-9 months vs 3 month 3 weeks 3 days of the G5. The only way around these limitations (it's a software issue in the receiver) is to use Xdrip or Xdrip+ and plan on replacing the transmitter batteries yourself. Sensors are readily available on Craigslist, EBay, Amazon, LetGo, and OfferUp - prices and use by dates vary. Best of luck to you in your quest.