The local airport is at Kalamata where several charter companies from the UK have flights each week in the summer months, usually May to October. Also EasyJet provide a direct service from London during most of the year. If you search for "charter flight" pages from the UK you should be able to find a reasonably priced flight to suit you. Car hire is available at the airport and the journey to Stoupa should take just over an hour. Taxis are also available that can either bring you direct or take you to the bus station from where a bus runs to Stoupa.
Flights to Kalamata also come from: Czech Republic, Holland, France, Sweden, Norway, and Finland.