I have exactly one 12U7 in stock. I have a few 12K5s,
as
well.
I've now got a number of different dual triode tubes to try from a local electronics
shop that has a large collection of tubes from former TV and radio repairmen. $1 each and
they have a B&K 707 tube tester to test them on.
Well, in case you wanted proper space charge tubes for your digital
logic experiments.