land
I will have a look into some land (lease hold) for you, whatever you do though, you will need to apply to the Local Authority for planning, as you were talking about netting. It'll be counted as a "change of use" for whatever the land was being used for. Even if its freehold you will have to do this.
And a paintball field might get opposition at the planning stage from anyone in the area, unless you were to make it 100% temporary. Planning will take up to 6 weeks for approval, you may want to pitch for (getting kids of the street) program they would really go for this.
I will have a look at the OS land register for the Manchester area, although narrowing down the area may help.
Out on interst what size (Acres) and prize range are you looking at.