Ive fitted alot of seats in the back of vans, used to be a truck and van mechanic.
If you're retro fitting them and it's a standard van floor (without seat rails) the best way is to buy the seats that have seatbelts built into the seat so no belt anchoring is required.
To fit the seats you need to get from a fabrication shop some 5mm mild steel plate cut into a minimum of 100x100mm squares, one for each hole on the seat base (normally 4 for a single seat) and drill one hole into each of the plates the same size as the bolt holes in the seat bases (normally 12 or 14mm holes). Drill the same size holes through the floor of the van (after measuring and marking out the seat locations).
Before drilling check where the fuel tank is, any electrical cabling and handbrake cables and make sure you dont drill through these!!! Also be carefull as on newer vans (the new shape vito for definate) have the wiring looms for the rear lights, doors etc running right down the centreof the back under the wooden floor!
Bolt the seats through the van floor and put the steel plates on the underside of the van floor. Use Nyloc nuts and don't overtighten as you'll crush the wooden floor lining, they do need to be tight though, obviously!!
You can get seats new from sites like this
http://www.jennings-seats.co.uk/shop/4/index.htm
but they are very exspensive.
I did a mates van for his band, put a seat and table in the back, carpetted it etc. we got the seats from a van breakers in manchester and got 6 seats for