Thailand forum

The majority of places on Ko Phi Phi cannot be booked online -- they work on a walk-in basis only. Your best tactic is to be on the first boat to the island on any given day and that will give you the best range of what's available -- be prepared to do some walking!

For Railay, you've got a couple of options. Most of the places on sunset side can be booked online and as you say are probably already jampacked across peak season. It's not all bad though, there's a bunch of places on the sunrise side that, like Phi Phi, cannot be booked online and rely on walk in only. It's a veritable bungalow village.

Lastly, there's Ao Ton Sai, the bay between Sunset Railay and Ao Nang -- it has stacks of places, none of which can be booked online -- again you'll need to do a lot of legwork, but as long as you're there early, you should find something.

Following stories may be of interest:
Do I need a reservation for my holiday?
Do I need a reservation for the Full Moon Party (while specific to Ko Pha Ngan, the advice is good for Phi Phi also).

Lastly, we've got a researcher doing a Phi Phi update at the moment - hopefully will be finished in a week or two, so watch this space!