The Greek National Tourism Organization (GNTO or EOT) has offices in Olympia and Patras. Nafplion has a municipal tourist office (Pl. Martiou 25. 27520/24444) across from the OTE (telephone-company) building. For tourist information and general help in places without tourist offices, it's best to contact the local tourist police, who often speak English and can be extremely helpful.
Because of the absence of Greek National Tourism Organization offices in the Southern Peloponnese, you will come to count on the tourist police for help and information. You can reach them by dialing 171 or local numbers. Beware of good-intentioned locals if you ask for directions to the tourist office—they may lead you to private travel agents (who can indeed be helpful) or to municipal offices that are not geared to helping the public.
Greek National Tourism Organization (Tsochas 7, Athens. 210/870-7000. www.gnto.gr). Olympia (Praxitelous Kondili 75. 26240/23100). Patras (Filopimenos 26. 2610/621992).