The device would have to be an original Virgin Mobile Device. You can NOT BYOD at this time.
You can shop for a device below. Also trading the device in at a Bestbuy or such works too. Sorry!
Before making any decission or spending more money, you should verify that your iPhone 6s Plus is fully compatible with our network. See, even if it's an unlocked device, there are some network parameters that could restrict your phone from activating with Virgin Mobile.
You may contact us privately, while also submitting your device IMEI number, so that way, we can check it and let you know what needs to be done in order to activate it.