From year end (about October/November) till about early February the following year, you would be able to find calendars sold in bookstores across Thailand.
This year, I noticed there are more varieties of calendars including those in English language. If earlier, you can find calendars being sold online as well.
There are various themes of calendars that are being sold…. including those featuring the glorious kings of Thailand and beautiful scenarios. I was a little surprised to see the Feng Shui calendar of Joey Yap, a popular feng shui master in Malaysia being translated into Thai language and sold in the SE-ED bookstore. I used to buy a Joey Yap calendar (Malaysian edition) for one of my former boss as a Christmas/ new year gift which she was able to place on her table and use for the entire year.
Aside from that, there are also various planners and notebooks sold for those who plan to stay organized for the following year.
During my annual visits in the past which coincide with year end, I would purchase a Thai calendar during my visits which contained various Buddhist observations dates and holidays. You can consider getting a calendar too if your visit to Thailand happened to be during year end/ beginning of the year.
Bookstores can be found in most shopping complexes like Central Plaza which have many outlets across Thailand.