The original and best way to get any kind of hunting permission is to knock on doors mate. Once you have permission from one farmer it is easier to get permission because you can go to nearby by farms and say "I was just wondering if you had anyone controlling your rabbits, as I control farmer browns and would be happy to help you out". Or that kind of thing anyway.
Even if 10 people say no just keep trying, sometimes the most unlikely looking places will just about tear your arm off once you give them the offer of free pest control!
Good luck!