The cheapest option is definitely a ferret and nets. If you want to get yourself an air rifle, can I suggest that you also find somewhere to learn to shoot it ! Go to a local club and practice, practice, practice.........you'll make friends and start to build up a network of contacts.
As already said, get yourself into a local shoot - be patient, do your time, and you can work your way up. You'll also make useful contacts into other activities.
I started out at my air rifle club, plinking away with a hundred quid air rifle...........now, I have six air rifles, two shotguns, five ferr