Nakhon Chai Air Bus company run an efficient survice to Chiang Mai from Pattaya. Located on Sukhumvit Rd near the Pattaya Klang intersection.
You can book a ticket on their website.
More info at www.yourpattaya.com/transport.htm
Air conditioned buses depart Pattaya for Bangkok and Bangkok for Pattaya every half hour between 5.30am and 11.00pm. The single fare is 118 baht per person.
There is also an airport bus service. For map, fares and time table go to:
http://www.yourpattaya.com
Here you can take a bus to Pattaya and other locations in Thailand. This is the closest bus terminal to the old Don Meuang Airport which is no longer in use. See details how to get to Pattaya from the new Suvarnabhumi airport here at www.yourpattaya.com/b