I think you should give your location or county, then you would get more local reccomendations.
I would say in general, the smaller garden centres with less overheads are much better value and would be able to give you a decent discount.
I go to lots of different garden centres and have found that the best value is to be had at a little place that has just the owner and one part time staff member.
Alternatively if you had a plant list you could check out any local nurseries. Or go to them and see what they have available and work from there.
good luck, i hope to be in your position some time this year (when we get the building/hard landscaping finished)