For carpet, especially if you want good quality and an unusual colour I suggest that you buy carpet you like and underfelt from a car trimmer. Cut the underfelt and carpet to fit the front and rear footwells and glue (Impact adhesive) each of the four pieces together (carpet to felt). While you are making the driver's footwell piece, decide whether you want the accelerator pedal in the aft or forward position (there are two pair of self-tapper holes) - Rootes' carpets assumed aft which is no good for long legs. The carpet has slots which fit around each pedal and the column.
The passenger's footwell should have a false floor (from SIII on).
Do the same for the small strip which covers the cross member behind the ash-tray, which has edging front and rear, and vinyl on the rear wall of the crossmember
Take the glued pieces back to your trimmer and get him to edge them - the correct way is to sew a 2" (approx) strip FACE DOWN onto the top surface with the vinyl flush with the edge and the stitching about 3/8" from the edge., then fold it back over the edge and sew it again, just inside the first stitch line. Also get the trimmer to fit the vinyl 'strap' and press-stud which goes over the steering column.
Just glue carpet onto thick card for the the inner sill sides.
The front wall of the rear seat down to the floor is just raw carpet, but with a strip of vinyl at the top.
The tunnel beneath the glovebox and the diff cover are also covered in raw carpet.
This leaves the gearbox cover. First cut felt to fit. The ashtray area is tricky. Cut away felt each side of the centre line from about 2" behind the gear lever, and also around the ashtray, leaving a 1/4" gap around the ashtray (see below). You can then stitch/glue joins in front of and behind the ashtray on the centre-line.
Once the felt is OK, do the same for the carpet and glue it to the felt while in the car. Cut and gather (sew) the carpet so it fits the ashtray area - but also fold it round and under the felt where it meets the ashtray and gear lever grommet.
You can't get it perfect (or at least I couldn't), but you can get it as good as Rootes' original. Finally, fit the 4 carpet press-studs which hold each front footwell carpet in place.
I can't help with a seat supplier, but note that you can buy new moulded 'stuffing' and diaphragms, at least for SIV and SV.