Best Option - go to Ekamai Bus Station and take the 999 government bus which are safe and reliable and do not speed. bus leaves at 7.45am and 9am - arrive bus station around 30 mins before and buy ticket and pick seat no need to book in advance. Bus will stop 2 times for food and toilet stop. Journey to ferry will take 6 hours and will cost around 300 baht each. Please check www.iamkohchang.com which is an excellent website for all info on koh chang.
No need to buy more than a day in advance. If you can't get the 999 bus Cherdchai Tours are the next booth along and they have a bus leaving every hour.