Cactus Mall lists many who have online seed lists, though some links may now be dead.
http://www.cactus-mall.com/seeds.html
As Spence says some seedsmen will not ship seed of cacti on Appendix 1(for instance Mesa Gardens) to the EU or other countries, but other species are no problem internationally and all on Appendix 1 can usually be obtained within the EU anyway. These travel with no problems across the EU's internal borders since the EU is treated as a single country for CITES regulations.
http://www.cactus-art.biz/note-book/Dic ... _cites.htm
Most seedsmen now take PayPal, so payment in different currencies is usually no problem.
