Can I not buy it out of season?
It is listed as Main season listed late Dec–Feb. The season can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Sweet Tamarind · มะขามหวาน
Try sweet tamarind, which is inside a brown pod. Open the pod and strip the fibres from the flesh, and you can eat the soft pulp. If you knew tamarind only as an ingredient in the tangy sauce of Pad Thai, meet the sweet tamarind eaten as a fruit too.

Main season listed late Dec–Feb
The season differs by growing area and variety
Sweet tamarind of Phetchabun: season listed as Nov–Apr
Use the main season, or the season of a regional fruit, as a guide. Actual availability can vary with growing area, variety and weather.
Sweet tamarind is a fruit eaten soft and sweet. Take out the hard stone and the fibres in the middle and eat the flesh.
Apart from the tamarind that gives dishes their sourness, there are varieties enjoyed as sweet fruit. The sweet tamarind of Phetchabun is introduced as a regional GI fruit.
It is listed as Main season listed late Dec–Feb. The season can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Tell apart fresh fruit in the pod and packaged products with the stone removed or processed. Even for the same tamarind, the product and pack weight differ.
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.