Can I not buy it out of season?
It is listed as Main season Mar–May. 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.
Mango · มะม่วง
If you want something sweet in Thailand, try a ripe mango. Varieties such as Nam Dok Mai are known for soft, fragrant yellow flesh. If you have had mango sticky rice, eat the fruit on its own and notice the taste of the mango itself. There are also kinds of unripe mango that are eaten crunchy and sour.

Main season Mar–May
Listed as available all year round
Nam Dok Mai Si Thong of Phitsanulok: season listed as Mar–Jun
Use the main season, or the season of a regional fruit, as a guide. Actual availability can vary with growing area, variety and weather.
Ripe mango has a soft, sweet taste. Some green mango varieties are eaten for a texture different from ripe mango, so you need not assume that every green one is unripe just by its colour.
Thailand has several mango varieties, such as Nam Dok Mai and Maha Chanok, and a food culture that enjoys ripe and green fruit in different ways. It is also familiar as the topping of a sticky rice dessert, so it is a fruit worth tasting on a Thai trip.
It is listed as Main season Mar–May. 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.
If you want sweet, soft mango, say that you are looking for ripe mango. Crunchy mango can also be sold separately. The price unit differs between buying whole fruit by the kg and buying a pack of sliced fruit.
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