Best Options for GearVR compatible phone without the phone plan?

Hi all,
I have an iPhone 6s plus on a Verizon plan here in the US.

I am very interested in exploring GearVR dev, but really don't need a new phone plan. I image I can buy a new S6 from somewhere, but I was interested in learning about the most economical options that folk have encountered in acquiring a compatible phone.

Any thoughts would be greatly appreciated.

Best,
B.