Can I only buy it in season?
The guide gives its timing as: Main harvest listed as late December–February. The harvest period can vary with the growing area and the variety. The calendar does not guarantee that it is on sale on any particular date.
Sweet Tamarind · มะขามหวาน
Try sweet tamarind from inside its brown pod. Crack the shell, take off the fibres clinging to the flesh and you can eat the soft inside. If you have only known tamarind as the ingredient for the sour sauce in pad Thai, meet the sweet tamarind that is eaten as a fruit too.

Main harvest listed as late December–February
Timing varies with the growing area and variety
Sweet tamarind from Phetchabun: harvest period listed as November–April
Use the main season, or the harvest period for regional fruit, as a guide. Actual availability can vary with the growing area, the variety and the 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 adds sourness to cooking, there are varieties enjoyed as a sweet fruit. Sweet tamarind from Phetchabun is presented as a regional GI fruit.
The guide gives its timing as: Main harvest listed as late December–February. The harvest period can vary with the growing area and the variety. The calendar does not guarantee that it is on sale on any particular date.
Tell apart fresh fruit in the pod from packed products with the stones removed or processed. Even the same tamarind differs in product and pack weight.
If you would like to read more about the food and the regions behind it, these sources are worth a look. Ingredients and cooking methods can vary from one cook to the next.