Can I not buy it out of season?
It is listed as Main season Oct–Dec. It is available all year round, but quantity and price can change with the season and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Pomelo · ส้มโอ
Try pomelo, which looks like a giant grapefruit. Peel off the thick rind and strip the inner skin, and the grains of flesh show. Sweetness and sourness feel different depending on the variety. If it is hard to use a knife at your lodging, you could choose pomelo that is sold prepared in a container.

Main season Oct–Dec
Listed as available all year round
Use the main season, or the season of a regional fruit, as a guide. Actual availability can vary with growing area, variety and weather.
It is a citrus fruit in which you can feel the grains of flesh. Colour, sweetness and sourness differ by variety, such as yellowish or pinkish flesh.
Thailand has several pomelo varieties, such as Khao Nam Phueng and Thong Dee. Large whole fruit is also sold prepared with only the flesh in a container, so it is an easy fruit to taste while travelling.
It is listed as Main season Oct–Dec. It is available all year round, but quantity and price can change with the season and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Tell apart the price per fruit, the price per kg and the price of a prepared pack. A whole fruit with its rind and a pack of flesh only are hard to compare directly at the same weight.
If you want to read more about these dishes and regions, these sources are a good place to start. Ingredients and cooking methods can differ from one eatery to another.
Sharing image credit: Ivar Leidus · CC BY-SA 4.0