Virgin Mobile iPhone, do I have to buy the phone from them?

You can now use an iPhone with Virgin Mobile service (plans starting at $35) but the phone itself is $$$! I'm wondering if you can buy a used iPhone or one off amazon and use it with a Virgin Mobile plan?
http://www.virginmobileusa.com/shop/cell-phones/iPhone4S-16GB-phone/features/

It has to be a Virgin Mobile Iphone. If you can find a used one then you could use that. The only restriction is that it is a Virgin Mobile iphone. So you can't get an AT&T or Verizon one and use it.

As for the cost you are not looking at it long term. Yes the upfront cost of the phone is high but that is because it is not subsidized. So you pay $449.99 for the phone and $35 a month for service. That comes out to $1289.99 for two years of service with VM.
With AT&T the cost for service alone is $70 ($40 for voice and $30 for data) a month. In two years you will pay $1680 just for service!