The rule of thumb is that developing regions feature a less-stringent control on prescriptions. India is an amazing country with a lot of great history and modern achievements as well. It's still considered a backwater compared to a country like the United States, especially when we're talking about your average person on the street in Mumbai as opposed to a more wealthy Indian.
My experiences in both Cambodia, Laos and Iraq were all very similar. Turkey was the notable difference in that you could feel the heat on you there in terms of the police, citizens eyeballing you and the knowledge of a draconian history of drug law enforcement. Turkey will fuck with you if you get caught as a narco-tourist. Anyway.
Both Iraq and Cambodia featured walk-in clinics that seemed to be popular for most sectors of society. There will be a waiting room, a dozen or so people, you'll pay 20 bucks, see the doctor, tell him what you want and then pay for the price of the pills. The places that I went to all had dispensaries attached to the building, so you would get prescribed and pick up your medications in one shot.
The thing is, I couldn't get a Morphine, Oxycodone or any other Opioid script. Another thing, Amphetamines. They wouldn't prescribe any kind of Amphetamine, although they would give you as much Methylphenidate as you could ever want. The most potent Opioid easily available in both spots would be either Tramadol or Codeine. At the pharmacies, you can get Benzodiazepines, Methylphenidate, Gabapentinoids and what have you, but for stuff like Morphine or Amphetamine, you're going to have to find some on the street,
It shouldn't be hard in India. My advice for doing this sort of thing anywhere unfamiliar is to work your way up. Ask first for Charras or something easy. The maybe ask about Opium, then move on to the Morphine/Heroin once some trust has been established. I would never advise anyone to go blindly in another country starting off trying to find Hard Drugs.