go and see your local farmers mate tell them that you would like to do a bit of field sport with your dogs and could help clear up a few rabbits at the same time. 
make sure you let them know that you are trust worthy and will behave yourself  
if they say yes ask for a peice of paper with written permisson and the signiture  
cracking your off mate  
i have not had much trouble getting permission as the farmers want rid of the rabbits  
  
i just got some more today as long as your honest normaly they are fine