Hi Hulskof, unfortunately, I don't know anyone in the Benelux who sells Ei, in Germany WVS-Technology sell Ei Elite 6DJ8EG at EUR 11 p/piece and you pay a small surplus for matching. However, they're not squeamish on "small quantity surplus" I'm afraid (EUR 25,-, yes sir). So, count approx. EUR 55 shipped for a matched pair.
http://www.wvs-tech.de/wvs-tech/data/index.php?page=/wvs-tech/data/produkte/audio%2520roehren/ei_e.php
Just an innocent reflexion: double that amount and you are square in the ECC88/E88CC NOS realm, so, should the tube roller bug have bitten, it may be worthwhile to talk with Mattijs de Vries at MachMat (e.g. pair gold-pin E88CC NOS Siemens for EUR 89, see http://www.machmat.com/sales/ecc88.htm) or Jac van de Walle at Jacmusic (e.g. pair gold-pin/gold-grid 1965 vintage E88CC Tesla for EUR 112, see http://www.jacmusic.com/nos/ECC88.htm).
Sorry, my enthousiasm is gaining the upper hand it would seem.
Anyway, this will give you an idea of prices in our small corner of the world; perhaps your audio dealer might get some Ei without the hefty surplus.
Good luck!
Karel