Can I not buy it out of season?
It is listed as a fruit available all year round. What a shop actually stocks, in terms of variety and quantity, still differs, so look at whole fruit and prepared fruit separately.
Young Coconut · มะพร้าวอ่อน
If you want cold coconut water on a hot day, choose a young coconut. Once the top is opened you drink the water inside, and you can spoon out the soft flesh too. The fragrant Nam Hom coconut is a known variety in Thailand. When you are thirsty, you can also share one with your companion.

Available all year
Listed as available all year round
Nam Hom coconut of Ban Phaeo: 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.
With a young coconut you enjoy the water and the soft white flesh together. Nam Hom is introduced as a variety whose character is its aroma.
The fragrant young coconut of Thailand is a fruit whose water you drink and whose flesh you eat. The fragrant coconut of Ban Phaeo in Samut Sakhon is also introduced as a regional GI fruit.
It is listed as a fruit available all year round. What a shop actually stocks, in terms of variety and quantity, still differs, so look at whole fruit and prepared fruit separately.
Coconuts are sometimes sold one by one. Bottled coconut water and a coconut that is opened for you are not the same product, so look at the two menu items separately.
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: Crisco 1492 · CC BY-SA 3.0