Developing services in MVC 6

I need to develop a MVC 6 service which currently uses MS SQL Express database as data store. Later on, it may access another existing service or any other flexible data source. I have my solution structure like this.

 Service->Business->Data Access. 

Later on, if I want to extend my code, where should I have the logic to access another web service.i.e, in which layer..?? I know I need to use dependency injection, but where the switch of data sources should happen..??