Can I not buy it out of season?
It is listed as Nakhon Nayok season listed Jan–Mar. The season can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Mayongchid / Marian Plum · มะยงชิด
If you see a small, yellowish fruit that looks like a little mango in Thailand, ask whether it is mayongchid. It is a fruit of the plum mango group, and the mayongchid of Nakhon Nayok is introduced as a regional GI fruit. If you want to try a fruit different from large mangoes, you could look for it in winter and early spring.
Nakhon Nayok season listed Jan–Mar
Season given for the regional fruit
Use the main season, or the season of a regional fruit, as a guide. Actual availability can vary with growing area, variety and weather.
The Nakhon Nayok mayongchid is introduced as having sweet and sour notes together with crunchy flesh. Take out the stone in the middle and eat.
The Tourism Authority of Thailand introduces the mayongchid of Nakhon Nayok as a regional GI fruit. You can learn about a small fruit that looks similar to mango together with its growing area.
It is listed as Nakhon Nayok season listed Jan–Mar. The season can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Even among similar plum mango types, the varieties and names differ. It helps to say that you are looking for mayongchid and get the actual variety and price.
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.