Getting Around the Mekong

Hi

Planning a trip around Laos and Cambodia but flying into Bangkok. I was just wondering how easy and cheap it is to fly around the regions once into Bangkok i.e is it easy to get a flight from Cambodia into Laos and is it quite reasonable?

I would prefer to book the flights when over there to try and save some money, but need to know if we can easily get flights for the next day etc.?

We hope to fly around mid August time.

Any help/advice would be appreciated.

Thanks