Most vacuum tube computers used extremely common pentode and dual triode tube types as their primary logic elements; ENIAC made extensive use of the 6SN7 for ex

Most vacuum tube computers used extremely common pentode and dual triode tube types as their primary logic elements; ENIAC made extensive use of the 6SN7 for example, which is still manufactured today for guitar amplifiers. One caveat is they were a subtype specifically burned in and selected for computer reliability, which is not done anymore obviously. For the types where that's not an option, surprisingly (at least to me), most tubes commonly used historically still have ample new old stock tucked away in warehouses across the US and Europe. There was no major problem finding all the tubes required to recreate EDSAC in the UK: https://www.tnmoc.org/notes-from-the-museum/2013/1/9/production-begins-on-recreation-of-edsac https://www.tnmoc.org/notes-from-the-museum/2013/1/9/product... for example.