After trying out a variety of store-brands, I decided to make my own sambar powder. Taking inputs from a variety of sources and trying out a few permutation-combinations, finally figured the below formula which works great for me. I don't think I'll ever buy sambar powder again...
Ingredients:
dried red chillies - 2 cups
bengal gram (chana dal) - 1/4 cup
black gram (urad dal) - 1/4 cup
fenugreek seeds - 1/4 cup
Method:
Dry roast all the ingredients separately until they give a nice aroma. The gram should just start turning light brown. Allow to cool and dry grind everything together. Store in an air tight container and it lasts a while (even for a few months).