Your best bet is one of the big Dutch shops, either Telstarsurf or Van den Berg. From what I've heared is that mXr isn't producing any new fins at the moment.
Telstarsurf does ship abroad and also has quite a large stock of 2nd hand mXr fins that aren't on the website. I suggest sending them an email what the possibilities are.
http://www.telstarsurf.nl/windsurf/vinnen/1/m/mxr/contact
info@telstarsurf.nlwww.vandenbergsurf.nl/windsurf/rigging-gear/fins.html?manufacturer=2608#page=1&top=84&contact
info@vandenbergsurf.nl