YG
Shindo has replacement tubes for all of their products.
When every model is sold, a fraction of the parts are used for production and another fraction is set aside for spares. When the parts for production are used up, the model production ends and something new is put out.
So don't worry about the tubes. If you want some backups to have on hand, you can find them on eBay or just contact Jonathan Halpern the distributor or Matt Rotunda at Pitch Perfect Audio.
Also, Shindo runs the tubes in their gear very conservatively so they should have a long life.