Upvote:5
Stamps can be purchased pretty much anywhere in Singapore: SingPost offices, obviously, but also "S.A.M." machines found at all larger MRT stations, most convenience stores and newsagents, and most souvenir shops that sells postcards, although possibly at a small premium. Even Changi Airport has a post office in Terminal 2.
A regular postcard from Singapore to anywhere in the world including India costs S$0.60, except Malaysia and Brunei, which are just S$0.40. Stamps are available for the exact price, or you can use two local mail stamps at S$0.30 each, equaling exactly S$0.60.
To mail your postcard, drop it in the "Overseas" slots of a SingPost posting box, which are even more ubiquitous: you've virtually guaranteed to find one outside an MRT station exit, and of course at a post office or next to a SAM machine. Your card should reach India in 5-10 days, although what happens at the Indian end is another story...