Dear Mab 1376,
Here is a link to German EBay for Siemens EK90 tubes (three offerings, but don't know about pairs - you have to ask seller):
To find these tubes when you go to EBay, look on the left and scroll down to the heading Location; then click on Worldwide.
Hope this is of help
And further down the page, under "More items related to "Siemens EK90" there is one listing with 5 available:
However, these look a little different than mine. Mine have ribbed plates, whereas, as best I can see, these do not. Mine also have a three digit number below EK90, and these do not. And we cannot see if these have O-getters. And further, we can't see the bottom of these tubes, but mine have an embossed number in the glass between the pins. All that said, even though they look a little different, they certainly look genuine,
Edit: Oh, my bad! That link is for EH90/6CS6, not the EK90!! Sorry!! (>_<)
Edited by gibosi - 6/24/13 at 12:10pm