Buttermilk is used in biscuits because of it's acidity as well as its fat and liquid content.
The buttermilk's acidity is used, in conjunction with leaveners, to help the dough rise in the biscuits.
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.