Getting to Bangkok by bus is not something that readily comes to mind, but there are some excellent services between Kuala Lumpur and Hat Yai, southern Thailand's traveling city that offer a level of comfort that can even out-do first class air flight. Then catch a bus from many transport companies at Hat Yai bus terminal, it's take 12 hours to get to Bangkok.

If you are strong enough to take a bus from Singapore to Thailand, you can buy a ticket at Golden Mile Complex at Beach Road in Singapore. It's a big, busy and confusing terminal near Lavender MRT station.