Sambar powder is an aromatic blend of a few lentils and whole spices. There are many recipes of Sambar powder. Most of them are made with coriander seeds, cumin and lentils as the base. In some regions of India, ingredients like coconut or cinnamon are added. Sambar Masala is used to make popular Indian dish - Sambar.
If you cannot source it, use equal parts coriander and cumin, and a touch of turmeric powder and red chili or cayenne pepper powder.