Best way is to do a land registry search. Get owners name and address and write them a nice letter.
Also drive about. Knock on doors and leave details.
Local facebook pages I think are the best, or actual noticeboards in tack shops. I've looked on gograzing.com, but most of the ads are horribly out of date and most people post wanted ads for grazing on Preloved (in my area, anyway). Needs round here far outstrip the supply, so it's a word of mouth thing, or riding up driveways if you're brave enough!