We managed to get two from twickets. Realised pretty quickly that the alerts are useless - they come through minutes later than the tickets are posted, by which point they have already been snapped up.
To get in there first, I set up an auto refresh on the live greenman feed: https://www.twickets.live/tour/green-man-festival-2019/1083682342486151168?territory=GB&_ga=2.205565014.418070370.1562923500-857970293.1562923500
Every 5 seconds, I had the page auto refresh using google chrome and this tool: https://chrome.google.com/webstore/detail/tab-auto-refresh/jaioibhbkffompljnnipmpkeafhpicpd?hl=en
Hope this helps...