What car is it for?
I recently did this and went from a standard 20″ to an 18″ and maintained the same rolling radius. All the info i found online from various sources.
If the car comes with a TPMS (gives you actual tyre pressure readouts) then you will have to fit them for MOT purposes. I bought a mew set to save fannying around with removing them from the summer set.
The continental (as in European) tyre and wheel websites have better information on alternative sizes, PCD, offsets etc as there is a much bigger market for them there.
I bought a brand new set of wheels, tyres and sensors for not much more than the summer tyres cost