After a LOT of searching, I’ve discovered one shop in the city where you can buy all sorts of baking supplies in reasonable quantities for baking at home.
Arife Lamoulde, at Crawford Market in South Mumbai near CST Station, is a treasure chest. At first glance, it will seem like nothing more than a roadside store. But a closer look reveals that you can buy all sorts of moulds, icing decorations, flavorings, extracts, cutouts,etc from this one place. Everything you need to bake or make chocolates of any shape and size you can get here. They even have delights like edible cake glitter (manufactured by Wilton and imported/smuggled from the US, so its definitely trustworthy) that will make your dish super special.
This shop is located very close to the famous Baadshah restaurant. Its phone number is 9833706896. The exact address is: Shop No. 556, Crawford Market, Mumbai – 400001.
They also have a branch at Andheri, near the Paaneri Showroom, very close to the railway station. This shop is also pretty good but doesn’t compare to the Crawford Market store in terms of varieties of items available. The phone number for this branch is 02226711726. Its website is: http://www.arifelamoulde.com/index.html
If anyone knows of other such stores in the city, I would be very grateful for the information.