Here are many very options for accommodation and cozy guesthouses on nearly all of the islands bays and you can book them on this website.
If you like to stay near bungalow resorts, like for example Salad Buri getting frequently running out of available rooms for the week of the Full Moon Party. Probably a good advice is to reserve a Phanganhas some of the best beachesanywhere in Thailand.
Koh Phangan is a palm fringed tropical paradise, protected by gentle sloping coral reefs and green clad mountain range behind the beaches.
Two of the most popular of the bays, Had Rinand Chaloklummost of the time are filled up with sun worshipers but there's also less visited, more hidden beachesand bays. Koh Panghan's monthly FullMoon Partyis the world's best known beachparty, with international DJ's from all over the world lining up to get the honor of playing.
Lying only 18 km from the northern side of Koh Samuiin the Gulf of Thailandis the island of Koh Phangan. Being generally much more quieter and relaxed than its neighbor, only with the exception of the FullMoon partywhere a estimated number of 5 - 10,000 visitors come on the island for the infamous full moon in Haad Rin Nok.
Koh Phangan would be to get tickets from Songserm Travel for ferry and express boats, buses, train, airplanes to Phangan. Or you can get an 1 hour flight by Bangkok Airways every few hours from Bangkok to Koh Samuiand take the catamaran speed boat to Koh Phangan.