Buttermilk is a very American thing but it's becoming very common in baking recipes and in particular cakes and bread. The reason for this is that buttermilk kind of lightens batters and dough due to the reaction of the acids in the buttermilk with the yeasy and gluten in your dough.
In Kenya we don't really sell buttermilk. However if you are in need, here are a few cheats:
1. Dilute plain natural yoghurt with a little bit of water
2. If you don't have yoghurt, take 1 pink of milk and add either 1 table spoon of table vinegar or 1 tablespoon of fresh lemon juice and let the mixture sit for 5 minutes. It will curdle but you can simply whisk this before adding it to your dough. And don't worry the curdled bits dissolve into the dough or pastry.
None of these recipes will make a buttercream as thick as a shop bought version but the effect on your baking will still be the same. And fear not, the acidic taste is still neutralized by the yeast so your dough definitely wont taste like lemon.