Can I only buy it in season?
It is listed as Main season March–May. It is harvested all year, but quantity and price can change with the season and the 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 well-ripened 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 to taste the mango itself. There are also kinds of unripe mango that are eaten crunchy and sour.

Main season March–May
Listed as harvested all year
Nam Dok Mai Si Thong of Phitsanulok: harvest listed March–June
Use the main season, or the harvest period of a regional fruit, as a guide. Actual harvests can vary with the growing area, the variety and the weather.
Ripe mango is soft and sweet. Some varieties of green mango are eaten for a different texture from ripe mango, so you do not need to assume that every green one is unripe just from its color.
Thailand has many 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 good fruit to try on a Thai trip.
It is listed as Main season March–May. It is harvested all year, but quantity and price can change with the season and the variety. The calendar does not guarantee that the fruit is on sale on any given day.
If you want sweet, soft mango, say you are looking for ripe mango. Crunchy mango may be sold separately. The price unit differs between buying whole fruit by the kg and buying a tray of sliced fruit.
If you want to read more about the food and the regions behind it, these sources are a good next step. Ingredients and cooking methods can differ from one place to the next.
Sharing image credit: Ivar Leidus · CC BY-SA 4.0