join basc and then you can tell the owners, especially golf courses, that you are insured... might turn a straight no into a yes, or maybe.. can be a waiting game, or pot luck, being in the right place at the right time!
what timmytree said about identifying the problem is spot on aswell, that's how I got my permission
make a point that its 100% free service and you want nothing in return, tell them you will be using a silent air rifle and you don't take any risks, and that the vermin are causing way more damage than you ever will. all that kind of stuff..
come across as sensible,