Where to buy Isopropyl (Rubbing) Alcohol In India online or offline ?

I guess you should get it from any local pharmacy. But, I am not sure whether it can be purchased without valid prescription. In fact, it is a pharmaceutical preparation and may have restrictions on selling it without prescription.

However, the term Rubbing Alcohol is used not only for the Isopropyl Alcohol in India. The term is used for Surgical Spirit as well ( maybe it has the same chemical composition). You can ask for it from the local pharmacy.
Acetone or hairspray can remove ink stains. But, be sure to apply it first on some non conspicuous part of your phone to check. It may destroy the smooth finish and texture. You can ask for Acetone from any science college nearby.

I think you can get at Medical stores ( only high end and well known ) and if you want to buy it in online you can try it from Amazon.

Currently, it is Out of Stock but you can keep an eye on that and also use buy hatke Chrome Extension in this product, so that you can get a notification whenever this product is available.

I checked online on sites like Appolo Pharmacy and it seems the Isopropyl alcohol is also used by the doctors and people who inject themselves(the one who use insulin injections). So it seems to me that this is going to be available to any local pharmacy. However there are chances that you can find that pharmaceutical person may ask for the documentation if you are buying in large quantity. It costs around 50-80 Rs in local stores.

Just a side note, if you want this only for the cleaning of the screen and other parts of device, then you may instead look for other liquid. I am sure you can find such cleaners at much easier way online.