Hi, Paul,
Did you find a booster? I may have one; not sure at this point.
I don't know if there are any rebuilders in Nederland or Europe, but there are in the U.S. Stephen-Allen's in Florida and Studebaker International in Indiana (& probably others) sell them, but require a rebuildable core in hand before they will ship a rebuilt booster. I bought a rebuilt one from S-A and it works well and looks good.
If I, or someone else, come up with a core, I assume that either SI or S-A will ship to you. The rest is obvious.
I'll let you know if I find one here.
--Dwight