Where do you live? If you're in the USA, go to the dairy section ~ heavy cream is in the refrigerated section. Most of the milk in the USA is separated. They take the cream and make cheeses and butter out of it. In fact, if you beat whipping cream enough, you will make butter. I don't think you can make this at home. If you live overseas where the milk is not separated, you can let it sit overnight in the refrigerator. In the morning, skim off the cream from the top and whip that. Hope that helps.
Tanya I noticed you are in India and I dont know if you can get it there..it is a milk product from cows....I would suggest you make American buttercream. I use 2 pounds of confectioner's sugar, 1 cup of vegetable shortening, 1 tsp butter flavor and a few teaspoons of water to the desired consistency. Cream Shortening until it is fluffy, add sugar and butter flavor. Whip until fluffy and add butter flvor and any other flavor you desire like vanilla, rum, lemon, cocoa etc. It will keep for weeks in a closed container.