Can I only buy it in season?
It is listed as Main harvest listed from late December to February. The harvest period 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 from inside its brown pod. Open the shell and remove the fibers clinging to the flesh and you can eat the soft inside. If you knew tamarind only as an ingredient in the tangy sauce of Pad Thai, meet the sweet tamarind eaten as a fruit.

Main harvest listed from late December to February
Timing differs by growing area and variety
Phetchabun sweet tamarind: harvest listed November–April
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.
Sweet tamarind is a fruit eaten soft and sweet. Eat the flesh and leave out the hard seed and fibers in the middle.
Apart from the tamarind that gives dishes their sourness, there are varieties enjoyed as a sweet fruit. Phetchabun's sweet tamarind is introduced as a regional GI fruit.
It is listed as Main harvest listed from late December to February. The harvest period can differ by growing area and variety. The calendar does not guarantee that the fruit is on sale on any given day.
Look separately at fresh fruit in the pod and packaged products with the seeds removed or processed. Even for the same tamarind, the product and the packed weight differ.
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.