Welcome to the forum. The stock tubes are manufactured for Marshall by Shuguang. You can order them from Marshall or find other brands such as Valve Art or Penta KT66s which are the same tube. Another option are the Golden Lions.
'When I Don't Have Anything Interesting to Play, I just Play Fast' But then who wants to hear 64 bars full of 32nd-notes except the douche-bag blowing them from the stage.