Why does soda bread call for buttermilk?
Buttermilk has a delicious flavor that is not at all approximated by lemon juice. 

It's true that lemon or vinegar with milk will clabber it a little and provide the acid that the recipe needs but the flavor will be distinctly lacking.

Get some buttermilk. It's inexpensive and has a longer shelf life than normal milk. It is also exceptionally easy to make. If you make it a standard part of your pantry you will find yourself enjoying life more (or at least pancakes.)