a spacer is basically a thick washer (with holes drilled out for the studs) that fits in between the rotor and the wheel. the problem is that the lugnuts will engage less threads on the studs. as you can see, you might also have to buy longer studs.
an adapter might be a better alternative. again it is a thick washer but it has openings to lock it onto the hub and will also have it's own set of studs. just imagine a ring with of alternating openings and new studs. there is no need to get longer studs.
the spacers are pretty cheap but the adapters are not. you probably can beat getting a new set of rims that properly fit the vehicle for it's intended use.
are you going with a street truck look where the wheels do not stick out past the fenders, or do you want an offroad-ish look?