Where can i buy garlic butter

The super market would be the easiest way to go. After all, they would stock a lot of different dairy products. However, I'd suggest finding a store that sells fresh dairy products. That advantage will be that the items will be fresh and the stocks will not be old.
You can make garlic butter at home. You can find recipes on the internet, just do a search. That'll save you the trouble of going to a supermarket or a local store