We used our local carpetright store! We selected the type and colour we wanted from the showroom, and a fitter came along to cut and fit it a few days later. He made an excellent job of it - took most of a day to fit into all the little corners. Total cost was around £200, for a 7 meter motorhome, from front of the cab to the toilet at the back! Still looks like new, two years later!
This is a good idea, having a fitter measure up and perhaps whip the edges. What I have not seen are carpets of the quality fitted by the manufacturers which are often very hard wearing and easy to clean.