You could give Costco a try? I was in there buying salt tablets this week and noticed they have a tyre fitting bay, I asked the guy if they can do better deals than Kwik-Fit and he recons they can beat them on price and quality.
Dont want to state the obvious here, just make sure they are proper load rated VAN tyres, not car tyres, cos if you are carrying lots of water/weight car tyres are not strong enough