Apparently there is a direct Athens to Stoupa once a day.
As you are looking for travel info for Stoupa the "Zorbas" site link posted above is a great resource and i'm sure you will get good solid advice from people who travel there often and know all the top tips and where and hows for the Mani..
Athens to Stoupa direct link is not available on the page below but i'm sure someone has mentioned it before in the forums.
Try the site. really nice helpful people there.
Additional info i knew i had seen it somewhere..
GREECE PELOPONNESOS - Villa Vacation Home rentals in STOUPA KARDAMILI KALAMATA
Stoupa is lucky to be situated on two pretty bays with sandy beaches with the 2407m high Taygetos Mountains in the distance behind (about 10km as the crow flies). It is also about 100km from Kap Tainaron, the southernmost point of continental Europe.
Vacation rental : house for 4 persons.Sea view, 7 mn from the beach. Many excursions : Epidaures, Mycenes, Grottes de Dirou... Direct bus to Athens. Kalamata or Athens airport(4,5 hours from Athens).