Hi @NatM, in India it is relatively difficult to find postcards. Apart from GPOs (General Post Office), most post offices do not sell picture postcards. And even in GPOs selections are limited and designs are unflattering.
All major cities have GPOs. For commemorative stamps, you can visit their Philatelic Bureau. They usually have good collection. Other post offices only carry definitive stamps. Buying stamps online from India Post’s website is a huge task on itself as the user-experience of the website is horrible and also in online you can’t buy single stamps, you can only buy fullsheets and miniature sheets. I recommend visiting Philatelic Bureau in any GPO.
Here are are a list of online stores where you can postcards. I recommend PostcardsVille and Postjunction.
As far as I know, in Chennai a bookstore called Higginbothams sells postcards. You will most likely find photographs postcards by VB Anand there, which are usually some of the most liked postcards from India.