Not easy - especially out of season. Sunsail go there - they may have some allocation they could sell you - otherwise try any other holiday company that may use it - thomson perhaps.
Best bet might be to find out which airlines go there (charter only) I think, then trace the holiday companies from that.
You may find Corfu easier to get tickets, then ferry or sail down from Gouvia
Sailing Holidays fly to Preveza for their flotillas, they also do bare boat charter now - whether they could give you advice, they must suggest something for their clients.
I've made the trip to Corfu, out of season when there were no cheap flights, by taking Easyjet to Athens then the KTEL bus. The bus trip is quite pleasant and the run from Athens to Preveza shouldn't take more than 3 or 4 hours. From memory, the cost of the bus to Corfu was about £15 single.
KTEL have a web site which shows details of their routes.
We booked a flight through Nautilus Yachting in May this year. Not only was it cheaper than Sunsail (last year), but they take bookings in advance - unlike Sunsail (who leave it to the last minute to publicise the empty seats).
We looked at the EZJet to Athens then Olympic to Preveza route, which can be cheaper. Problem is to get the PM Olympic same day connection you have to get the EZ flight that arrives at 0400! If you catch the later EZ you need a hotel - which then defeats the object. Olympic cheap tho, around £40 return! Taxi takes 5hrs at around £50-60.
Another alternative is to fly to Kephalonia then grab a ferry.
Flights to Preveza by Charter Start April end October.Olympic via Athens to Prevessa or Ioannia and then taxi.Via corfu then ferry to Igouminisa and bus or taxi About 50 Mile.Athens to Prevessa by bus or taxi 5 to 6 hours depending on the rio ferry