@floriankaulich No, you are probably right. The data provided by Google Scholar is often not as complete as we would like it to be.
So, if someone is willing to implement a web search function in JabRef, I think we would include it.
Looking at the repec website though, I have to say that I have a hard time finding an actual search interface. There are several services listed, but they seem to be on separate data sets? Where do people actually search repec data? What URL do they use?