I (and many others here) have often used Mr. Valve, you can get hold of them via the web:
http://www.mrvalve.co.za
Their service is good and I've usually got my shipment in a day or two.
Otherwise, many music stores (e.g. Tom's in Sandton) keep some tubes in stock, though it's best to check if they have what you're looking for before making the trip.
