I am looking for a module or method to inject banner ads (these would be direct sales banner ads, sold and placed by the site owner, not Google Adsense ads, etc.) into a teaser list at a certain spot in a teaser list (for example, the default front page when Articles are promoted to the front page). For example, I would like a banner ad to appear after the third teaser listed on the front page.
This seems like a desired function, but I cannot seem to find a module or even an example function that would provide this. If anyone has any ideas on where I could find something like this I would appreciate it.