Replacement Tubes

It seems the OP has not looked very far! 6550's are available, and in stock at many places.

I can't reccomend VivaTubes highly enough. I ordered a matched set of 4 6H30Pi and they arrived at my door in just 5 days, and I'm in the UK! 6550's are available almost everywhere - I easily bought a pair of Sovtek 6550WA for my Ref3, although I only needed one, but singles are a little thinner on the ground and he wouldn't split a matched pair.

My supplier in the UK told me Russian tubes are rapidly going out of stock though, and they cannot get any more in the current climate, so I'd buy an extra for the future.

You can buy Sovtek 6550 tubes on Amazon....they have some in stock now.

I've bought tubes on Amazon and they worked just fine.

A few days ago I bought a Svetlana 6550 tube from the Tube Amp Doctor in Germany.  

I paid $150 total including shipping for it...it got here in three days.

+1 @larsman 

Go wild. They test them too. Ask ‘em. 

fan of the tube store.  ordered a pair of 12ax7s for phono preamp.  called first to consult, as I wanted lowest noise tubes available.  was steered towards a product that was well below their highest priced tubes.