Mr Valve is a great supplier of valves, if you want to beat the rush, send them an e-mail and order some tubes and you will most likely get them within a week.
http://mrvalve.wordpress.com/. If it is the power tubes that are the problem, you need a pair of EL84s - but it might be an idea to revalve the whole amp. You will then have spares for the ones that are still alright (so 3 additional 12AX7s/ECC83s - these are plug and play).
I guess the sales people assume that you would know that you have to replace the valves in a valve amp periodically. It also seems that this amp is a bit of a mission to bias, but that is still no excuse for them to have been so uncommunicative, slow and unhelpful.
It's important to note that if you buy your own set of power valves, you should have them biased by someone who knows how to do it for best results regarding tone and durability.