Stoupa has never been more accessible. However, like with most of mainland Greece, the journey time can be quite long.
Currently you can fly by Jet2 or EasyJet from most UK airports into Kalamata Airport, then its either a coach transfer, taxi or hire car down to Stoupa.
If you’re already in the Mani region, there’s a direct bus service from Kalamata to Stoupa operated by KTEL Messinias, departing twice daily, but it can be unreliable.