I go for the big ones people say are "impossible".
One tenant farmer said to me "If he gives you permission I`ll buy the cake".
Took 4 years but I turned up at the farmhouse with a cake and said to the farmer "Pop the kettle on".
I also ask random people who are working in the fields. Mostly contractors around here but most will give up a number for a bottle of Scotch. Ring landowner and start of with "Hello Mr ****, I know Tom **** your field contractor and he gave me your number......"
Then I also ask friends, family and wider networks.
So far I have permissions in Leicestershire, Rutland, Staffordshire, Shropshire, Essex, Wiltshire, Lincolnshire, Nottinghamshire & Devon.
One or two are "exclusive" due to paperwork arrangements with land agents, but the majority are available to anyone willing to do the legwork.
Warning: It becomes habit forming after a while.