Can I only buy it in season?
It is listed as Main season October–December. It is harvested all year, but quantity and price can change with the season and the variety. The calendar does not guarantee that the fruit is on sale on any given day.
Pomelo · ส้มโอ
Try pomelo, which looks like a very large grapefruit. Peel the thick skin and remove the inner membrane and the grains of flesh appear. Sweetness and sourness feel different depending on the variety. If it is hard to use a knife where you are staying, choose prepared fruit that is sold packed.

Main season October–December
Listed as harvested all year
Use the main season, or the harvest period of a regional fruit, as a guide. Actual harvests can vary with the growing area, the variety and the weather.
It is a citrus fruit where you feel the individual grains of flesh. The color, sweetness and tartness differ with the variety, such as yellowish or pinkish flesh.
Thailand has several pomelo varieties, such as Khao Nam Phueng and Thong Dee. A large whole fruit is also sold prepared with only the flesh packed, which makes it an easy fruit to taste while traveling.
It is listed as Main season October–December. It is harvested all year, but quantity and price can change with the season and the variety. The calendar does not guarantee that the fruit is on sale on any given day.
Separate the price per fruit, the price per kg and the price of a prepared tray. Whole fruit with its skin and a tray with only the flesh are hard to compare directly at the same weight.
If you want to read more about the food and the regions behind it, these sources are a good next step. Ingredients and cooking methods can differ from one place to the next.
Sharing image credit: Ivar Leidus · CC BY-SA 4.0