It looks like only Cambodia Angkor Air is the only one who flies into sihanoukville (from Siem Reap). It looks like you have ZED's on Bangkok Airways and Thai Airways so ZED into Phnom Penh and take the bus (or taxi) from there. Here is some information on the bus. bus 1, from tales of Asia and from wikitravel.
I can not help much with that area as I have only been to Siem Reap.