go on the estate agents web sites, if you are in arural area you will find some estate agents who sell/rent out farms,fields,etc. get them to send you their lists. ALso its worthwhile contacting the local council and also utility companies eg the electric board. they often have fields here and there, I have rented from both the council and the electric company in the past. another way is to put up adverts for filed wanted to rent in the local newsagents,supermarket and agricultural merchants. other than that its a matter of going around farms and knocking on doors, the answer is often No but if you leave a card with your name address and phone number on it and what you are willing to pay, after the farmer has thought about it he /she may ring you back.
I also knocked on a door - the land is at the back of a house so just knocked & asked!! I have to go through his garden everyday but it was so worth being cheeky