That depends on your budget, but if you've never tried VR before and have a smartphone with a decent screen, almost all of the sites include a free Google Cardboard headset that you can use to test things out. It doesn't provide the best experience, but it is a free way to see virtual porn without buying a headset first. If you see the potential of, say, connecting a headset to a fucking machine in the future, you can always upgrade later. If you have a Samsung phone, and don't want to spend much money, you can normally find a Samsung Gear VR for sale second-hand.