Hi Niku2,
It was AGESSS ago but back in 1986 or so, I remember driving up to Brum and picking out a carpet and underlay from a large warehouse there. Their prices then were a good 30-40% less than the high street stores we looked at. I dont remember the actual pricing and all that, i was only 14 or so, but I do remember being stuffed in the car with 2 rolls of carpet on the drive home!
I remember visiting some friends who were at Aston university about 93 ish and sort of remembered that warehouse, 'I think' it's around the area and for some reason, although I could be entirely wrong. It's that often I need to visit Brum itself and usually i'm happy just driving past it on the M6 or M42 !!!
Much more recently, we refurbed our house completely and the only carpet we have is in the living room and stairs/landing. A good tip for you, even though it increases the expense quite a bit, I told the carpetright people ( we didnt have time to go to warehouses this time) that I want the thickest underlay available for the ultra deep pile carpet we chose. Once it was fitted, the springyness and that 'sink' into the carpet as you walk feeling is fantastic! - if you're able to, try doing the same!
For the warehouse, try a local Brum search on local listings on the web - I do seem to remember that the name of the place was a 'Muslim' named place and 'sons' I think - something like Abdullah and sons Carpets or something.. sorry to be vague, but I always am when it comes to most things Brummy!

Hope this helps!
Cheers DtM! West London & Slough UK!