Travel by bus is one of the ways to get you around Thailand. You can go to northern, southern, eastern, or western of Thailand by bus. If you start from Bangkok, there are three main Bangkok bus terminals that you can buy a ticket and catch the bus: Mo Chit (Northern Bus Terminal), Sai Tai Taling Chan (Southern Bus Terminal), and Ekamai (Eastern Bus Terminal).
When to book a bus ticket to travel around Thailand? Whether booking the bus ticket three months in advance or on the day of your travel, you will get the same price. Generally, there is no discount or promotion for booking early.
So, no need to book early? Well, I would say it depends. If you travel during non peak period or long holidays in Thailand, you can get a ticket on the day of your travel. Well, to be safe, I recommend you to get it at least a day before traveling.
So, when are long holidays in Thailand? You can check the post of public holidays and long holidays in Thailand here. If you travel during those holidays, to get the ticket you are likely to book it a month or two in advance. In my opinion, if you visit Thailand during long holidays here, I do recommend you to consider flying. There are many budget airlines that fly all around Thailand.
Is it possible to book a ticket by not going to the bus terminal? There are many companies running a bus tour. For some companies such as NakhonchaiAir, you can reserve a ticket through call center (call 1624) and pay the fare at 7-11 convenient store, and also can book a ticket online through its website. Both options might not work well, if you do not know Thai. You can give it a try. If it does not work, I recommend you go to the bus terminals to buy a ticket.
Hope it helps. Enjoy Travelling Thailand!