US => Europe
is very tricky and full of bureaucratic traps (virtually impossible ==> France, relatively easier ==> UK & Germany). I know Sam Tsui (Orchid Inn--highly recommended) brings plants when he comes to the UK (at least once a year--he then forwards his plants to France or the EC without any hitch). You can ask him. He packs the plants to perfection and they arrive in France within 2 days (he usually visits in November & June, November wd be perfect to send plants to Thessaloniki, June may be too hot).
Perhaps you could also try to order from Germany (many German growers import plants from Taiwan which are among the very best at the moment). If you PM me I can send you the addresses of some reliable growers/importers. Since Greece is a member of the EC, there is no problem whatsoever--no phyto, no CITES, no import permit.
I strongly advise against importing plants on your own from a country which does not belong to the EC--I tried and it is a nightmare. Not only because of the red tape involved (CITES & phytosanitary certificates, import permits in n-plicates, etc.) but also because the bureaucrats will consider you as their personal enemy since it gives them WORK to do.
Thessaloniki is a superb city and we fondly remember Olympias where we used to spend the summer many tears ago before the tourists discovered the place.
Happy growing