The best place to start is to do your own research, check out Youtube channels and videos related to HomeAssistant, Finalize the technology you want to use for your IoT devices e.g., Zigbee, Z wave, WiFi, Bluetooth, etc., then go about finding those devices in India. Currently, I believe the best option of having HA integration with switches is Sonoff Mini's, but I believe Wipro just launch their Shelly competitive product in India, I don't know whether it is compatible with HA or not but if it's not, I will soon be as there are other Wipro products compatible with HA, but the Wipro items are a bit on the costlier side.
I exclusively used Sonoff Mini R3 in my home for Light control, I don't have any other IoT devices, other than Google Home speakers. Sonoff is largely available on Amazon, but Sonoff has its own website for India (Sonoff.in) where you can get their products, you can contact them for a quote if you buying in bulk, and they usually give good discounts online pricing.