For buying and developing B&W film in Thessaloniki there are many stores, but AFAIK these are the biggest ones:
Repellas:address Aristotelous 28,
repella@otenet.gr, tel +302310 231732 ,
http://www.repella.gr/fotografika/index.php?option=com_content&view=featured&Itemid=469
Kogxylakis: address Venizelou 42,
info@photoqueen.gr ,+302310 261771 / 2310 234842,
http://photoqueen.gr/index.php?route=common/home
Unfortunately the sites are in Greek so i am not sure how much hep it would be. in any xase the are both down town in very central part of Thessaloniki and not more than 5 min walking between them so it would be easy to find with a map and asking around.
Personally I go to the first one, but either is fine I think. They both do B&W film but they may need a few days to gather a number of films before going to the darkroom.
Available films found around here, are mainly Kodak (Tmax 100/400, Tri-X 400, chromogenic) and Ilford (FP4 PLUS, HP5 PLUS), and Agfa. If you are looking for high ASA film you may or may not find it.
If you have any questions feel free to ask either here in the thread, or by p.m.
Hope this helps