Buttermilk and Heavy Cream are slightly similar but they are not the same thing.
Traditionally, buttermilk is the liquid left after churning butter - milk, with little specks of butter left over giving it a slightly sour taste.
The buttermilk you would buy in the supermarket though will typically be low-fat milk with a lactic acid bacteria added to it to help sour it.
Heavy cream, also known as heavy whipping cream, is the thick part of the milk that rises to the top due to its high fat content. With about 36-40% fat, it has one of the highest fat contents compared to other dairy products.
Whipping cream comes in at a close but lighter second, with about 30% milk fat.
Heavy cream is used to add fat.
Buttermilk doesn't whip and has a much lower fat content than heavy cream (meaning it makes stuff lighter and fluffier than heavy cream does).
With a bit of know-how, you can substitute buttermilk or heavy cream for each other in a lot of cases.
Buttermilk in the grocery store is located in the same refrigerators as the regular milk is.
The milk carton or milk jug will say on it that it is buttermilk.
It should be located in the next refrigerator over from the regular milk or it may be located below or above the regular milk.
Buttermilk should be sold in the grocery store and located in the milk department or dairy department of the grocery store.
If you look where the gallon jugs of regular milk are in the fridge part of the grocery store then the buttermilk should be near the regular milk.
Buttermilk is sold in 1/2 gallon jugs, smaller jugs, gallon jugs of buttermilk and even some cartons.
The buttermilk in my local grocery store is right next to the regular milk.
If you can't find the buttermilk you can ask a worker in the store and they can show you where the buttermilk is.
But as long as you look in the milk aisle then you should locate the buttermilk.
It may also be in the same fridge area as the regular milk and could be on the lower shelf below the regular milk as well.