The problem is, if you want to fit them in a van as intended, then you need the runners and rails and you will need to buy some seat belts. Its these bits that are harder to get - we regularly throw a seat like those in the photo in the skip, but keep the runners etc to use as combi conversions.
If you buy without runners etc, then you could fix down permanently, but would need to raise the seat up a couple of inches as they are a bit too low even when put on their runners for a VW.
If your in the southwest and need the bits, then you could try Bongobits in plymouth. They break a few bongos and keep a stock of second hand parts.