I do not believe there to be any easy way to find the URL for specific ads. You may be able to sniff them out if you look very closely at your data traffic.
However, I would advice trying the Ad review center on AdMob:
- "Monetize" -> "All apps" -> "Ad Review center"
Once enabled and in "Run ads immediately" mode, you will be able to monitor what targeted ads are shown through your app, and can block the ones you don't like from being shown again. You can either block individual ads or entire AdWords accounts from appearing in your app.
You could also use "Hold placement targeted ads for 24 hours so they can be reviewed", but this only lets you review placement-targeted ads, while the previously mentioned mode works for contextual, interest-based, and placement.
Please note that these ads are blocked for all your apps, not just one. This information can be verified at Google Support.