guidothepimmp You can buy from tonetubes.co.za Reliable and safe. Many of us have bought from him before. The other alternative is approaching an amp tech in your area and see if they sell valves.
V8 +1 for tone tubes. If in Cpt you can also chat to the guys at Gear Junkie - the often have N.O.S tubes and can get some things. Big bonus is that they have a tube tester and will happily check out your tubes for you.