Angelfish are freshwater fish, yes, so you should be able to find some. Do be aware they need a large tank (min 20Gal I think) as they become very tall and are greedy at feeding times! Here's the profile for them: https://www.fishlore.com/Profiles-Angelfish.htm As for where you can get them, I'm afraid I wouldn't know in India, but perhaps you could run a search through Google or another search engine to find local angels.
Bettas may look peaceful but when another fish is placed in the same tank, then it's asking for disaster. So I'd also recommend to have a betta in his own tank minimum 5 gallons. If your country doesn't have a cold season, then a heater is not a must as long as the temperature doesn't fluctuate.
Hmm...thats an odd one. A betta and an angelfish together. I wonder, if disaster did strike, who would end up winning? I know some bettas can be quite submissive, at least for a little while. I put my one betta in with some goldfish and neons for a while, and they were ok. But than I got worried for nothing and put my betta back in his old tank where he was comfortable. My betta lived for 5 yrs!!