I've run into this problem many times. If it's owned by a timer company, I would go to their district office and talk to them about what you want to do. They will have an office somewhere within a reasonable distance. Asking in person always works better than a letter. If it's privately owned, nearby neighbors may know, or you can find out through locating the owner through tax records, etc. at the county courthouse. Again, I would ask in person, so they can better be aware of you and your intentions. Unfortunately, people are very concerned with liability nowadays, and that poses an possible problem. If you knew descendants of who is buried there, they stand the best chance of gaining access. Good luck.