12Go is an online travel agency where you book trains, buses, ferries, transfers and flights. The start page offers a search with a return option and a field for the number of travelers. Popular routes in Thailand are listed with starting prices, for example Bangkok to Pattaya or Bangkok to Chiang Mai. The site also advertises a free app with a 4.7 rating.
Search the route you need and compare departure times before you pick a ticket. Codes and offers that we find are listed here.