I buy tom yum paste in jars imported from Thailand at the Linda Vista vietnamese supermarket.
It makes good soup, but can also be added to stir fry.
It is a miracle product for me, one spoonful gives anything a Thai flavor. It is a mix of chili, lemongrass, ginger, and onion. To make a “tom yum sauce” for something like chicken, take the paste and mix with an equal part lime juice, or to reduce the intense flavor a bit, further mix with tomato paste or heinz chili sauce.