Matamp do sell valves, but they don't allways have matched sets if that's important to you (individual bias on most of the matamps mean you don't need matched valves.)
Usually they have JJ EL34s, JJ ECC83s, Sovtek or JJ 6V6s and 6550s, sometimes there aer others in stock.
Another good supplier is Edicron who import and distribute a lot of the brands. (they supply lots of companies like Maplin, Matamp, I think Watford too)
Valve prices could be on the up due to some political nonsense between New Sensor and Ivan....