If you believe that you can find a forum where people are spewing venom on your competitors, you will get none because it will against their terms of service and it will be reported and deleted. If you find it there it will be for a short term. No forum wants to have a bad name and fall in the whirlpool of lawsuits for allowing such posts. As far as using Google Display Networks are concerned this link will help you: https://support.google.com/google-ads/answer/2404190?hl=en
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath