Can I only buy it in season?
It is listed as a year-round fruit. What a stall actually sells, including variety and stock, still varies, so compare whole fruit and prepared fruit separately.
Young Coconut · มะพร้าวอ่อน
If you want cold coconut water on a hot day, pick a young coconut. The top is opened for you, so you drink the water inside and can scoop out the soft flesh. The fragrant nam hom coconut is a well-known variety in Thailand. When you are thirsty, it is also fine to share one with a companion.

Available year-round
Listed as available year-round
Ban Phaeo nam hom coconut: harvest listed as year-round
Use the main harvest period, or the harvest window for a regional fruit, as a guide. Actual availability can change with growing region, variety and weather.
With a young coconut you enjoy the water and the soft white flesh together. Nam hom is introduced as a variety known for its aroma.
Thailand's fragrant young coconut is a fruit where you drink the water and eat the flesh too. Aromatic coconut of Ban Phaeo, Samut Sakhon, is also introduced as a regional GI fruit.
It is listed as a year-round fruit. What a stall actually sells, including variety and stock, still varies, so compare whole fruit and prepared fruit separately.
Coconuts are sometimes sold one at a time. Coconut water packed in a bottle and a coconut opened for you are not the same product, so look at the menu items separately.
If you want to read more about the food and the regions behind it, these sources are a good place to start. Ingredients and recipes can vary from one restaurant to the next.
Sharing image credit: Crisco 1492 · CC BY-SA 3.0