Yes, you have to use their phone because of their 'hybrid calling' setup with wifi and cell. They currently have two new phones you can buy. One is $150 and the other is $300.
If you can find a Motorola Defy XT, you should be able to have them activate it on your account. That is their old phone, and the phone I am using now. They recently announced that you can transfer them to another account or reactivate them on a new account. I just checked ebay, and there are a bunch selling for less than $50. I just searched for "republic wireless".
republic wireless | eBay