Use any available sambar powder. Or just use 1 tsp coriander powder, 1 tsp(or more) chilli powder, ¼ tsp roasted jeera powder, ⅛ tsp pepper powder.
Small onions taste best.. But if unavailable, you shall replace with big onion.
Adding asafoetida at 3 different stages renders nice flavor to sambar. Pls do not skip it.
You shall make the same sambar using any mixed vegetables like carrot, beans, potato, chow chow(Chayote squash).