Take a Preveza to Athens bus get off at Rio on the Peloponese side of the new bridge across the gulf of Corinth, then take a local bus from Rio to Patras (a few miles away). Then a bus from Patras to Kalamata.
Check the Preveza-Athens bus does go thru/stop at Rio, check in Preveza KTEL.

You can usually jump off the buses passing through Rio near the toll booth as you get on to the road to Athens.

This means that you then have to find a taxi to get into Patras. However some buses passing through Rio drop you by the old ferries where there will probably be taxis waiting and buses too. I would advise talking nicely to the driver before you set off from Preveza so he can drop you somewhere sensible.

Hoping for a bus and praying for a transfer is like fairy dust!!!! The best/easiest/quickest way is to fly to Athens from Preveza, and then fly to Kalamata...it might be a little more expensive, but is the best/easiest/quickest......