Can I only buy it in season?
It is listed as Main harvest Mar–May. It is a year-round fruit, but volume and price can change with the season and variety. The calendar does not guarantee that it 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, try the fruit on its own to taste the mango itself. Some kinds of unripe mango are eaten crisp and tart.

Main harvest Mar–May
Listed as available year-round
Nam dok mai Si Thong of Phitsanulok: harvest listed for Mar–Jun
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.
Ripe mango is soft and sweet. Some varieties of green mango are eaten for a different texture from ripe ones, so you do not need to assume that every green mango is unripe.
Thailand has many mango varieties, including nam dok mai and maha chanok, and a food culture that enjoys ripe and green fruit in different ways. It is also familiar as a dessert served over sticky rice, which makes it a good fruit to try on a trip to Thailand.
It is listed as Main harvest Mar–May. It is a year-round fruit, but volume and price can change with the season and variety. The calendar does not guarantee that it is on sale on any given day.
If you want sweet, soft mango, say you are looking for ripe mango. Crisp mango may be sold separately. The price unit differs between whole fruit bought by the kilogram and a pack of cut fruit.
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: Ivar Leidus · CC BY-SA 4.0