Don't think you'll find the prices or expertise, associated with Cougar or Novatech, but a place I use a lot is Lynx computers, which is situated under the bridge on Argyle Street. (outside Central Station)
I've been to a few shops in Glasgow and bought stuff from them, never had a problem but have to say that in my opinion they cannot compete with the likes of Novatech. Its been so long since I bought from anyone in Glasgow I cannot even say if they are still in business, and I have not bought a complete system from them and since I have never had a problem either I cannot say if they have good aftersales or not. It has to be said that its differcult for a local computer shop to compete with the big companies as the big companies buy in bulk and are therefore able to sell cheaper and still make a profit.