I think you could start with this paper (Jansen et al., 2006). First, it describes the issues you wan tot investigate. Second, it contains a validated questionnaire for explorative and exploratory innovation. This might fit your notion of external and internal innovation.
I would also suggest to look for "incremental" (close to internal) and "radical" (close to external) innovation. These types of innovation are differently named but might be almost the same variable you are looking for.
I guess R&D investment and patents are good proxies for internal and external innovation respectively. Have a look at - Ahuja, G., Lampert, C. M., & Tandon, V. (2008). Moving beyond Schumpeter: management research on the determinants of technological innovation. The Academy of Management Annals, 2(1), 1-98.