I am looking for an online resource (paid/non paid) that can provide me with intelligence on specific iOS app download numbers over X period and revenue over X period.
A number of app developers have shared a few of their experiences, metrics and numbers on Quora, so that would be my first suggestion. Denys Zhadanov and his impressive team at Readdle also publish stories on their company blog highlighting metrics that might be very useful considering their apps are almost always in the top 100.
Beyond that, I'd also suggest having a look at App Annie, which offers a continuously growing source of indexed app store / marketplace data for all apps. They also have a number of paid products and content depending on your need for competitive intelligence and insights on your own apps.