Can I only buy it in season?
It is listed as Main harvest listed late Dec–Feb. The harvest period can vary by growing region and variety. The calendar does not guarantee that it is on sale on any given day.
Sweet Tamarind · มะขามหวาน
Try sweet tamarind, which comes in a brown pod. Crack the shell and pull the fibers off the flesh and you can eat the soft inside. If you have only known tamarind as an ingredient in the tangy sauce of Pad Thai, meet sweet tamarind eaten as fruit.

Main harvest listed late Dec–Feb
Timing varies by growing region and variety
Phetchabun sweet tamarind: harvest listed for Nov–Apr
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.
Sweet tamarind is a fruit you eat soft and sweet. Remove the hard seed and fibers in the middle and eat the flesh.
Apart from the tamarind that gives dishes their sourness, there are varieties enjoyed as a sweet fruit. Sweet tamarind of Phetchabun is introduced as a regional GI fruit.
It is listed as Main harvest listed late Dec–Feb. The harvest period can vary by growing region and variety. The calendar does not guarantee that it is on sale on any given day.
Tell apart fresh pods from packaged products with the seeds removed or processed. Even with the same tamarind, the product and the package weight differ.
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.