There are lots of brands available in India. Many companies which manufacture spices are now a days manufacture Sambar powders too. But it is always better to make it at home and freeze it in ziplock bags.

Coriander Seeds - 500 gms

Red Chillies - 500 gms

Thuvar Dal - 100 gms

Bengal Gram Dal - 100 gms

Black Pepper - 150 gms

Fenu Greek (Methi seeds) - 50 gms

Hing or Asafodeita - 50 gms 

Fry all the ingredients in a dry pan one by one till they loose their raw smell and turn golden brown. Let them cool, mix it well and grind it in flour mill. Store it in air tight container.

To avoid the smell of the red chillies in the house, dry fry all the other ingredients except red chillies and hing. Add a tbsp of oil to the pan and fry the hing first and then the chillies. Once the powder is ground, there will be heat in it and immediatly after reaching home spread it in newspaper and let it cool. Store it in zip lock covers and keep it in freezer.  Keep the necessary  quantity in an airtight container and keep refilling it. If you are making very less quantity use the same proportion but reduce the quantity and it could be grind to a fine powder at home in mixie.