I pulled this out of Sabre, but if you do a test booking on Expedia there is a link for Rules and Restrictions during the checkout process which has links to show you this info. It's definitely buried, and honestly, I just pulled up Budget as an example and it had different info than Sabre:

CROSS BORDER TRAVEL - ALLOWED FOR AN ADDITIONAL FEE (20.00 EUR PER RENTAL PLUS TAX INOT AUSTRIA, BELGIUM, CR)OATIA, DENMARK, FRANCE. FINLAND, GERMANY, GILBRALTAR, HOLLAND, HUNGARY, ITALY, LUXEMBOURG, NORWAY, PORTUGAL, SLOVAKIA, SLOVENJA, SPAIN, SWEDEN, SWITZERLAND AND POLAND. . NO CROSS BORDER PERMITTED TO GREECE, MACEDONIA, BOSNIA, TURKEY, ROMANIA, BULGARIA, ALBANIA AND ALL COUNTRIES OF FORMER RUSSIA.

Bottom line, I would call the rental company directly to confirm anything you see online. The bigger/better companies often have dedicated international desks that are better versed in this stuff than the general agents.