Hey Roger -
Stop by the County Clerk's or Recorder's office (whatever office is responsible for Deeds, Leases, etc) at the County Courthouse and ask if anybody using the records is a Landman.
Take along your legal description and a map of the land if you have one (Tax or Appraisal Office should be able to print you out both).
Unless they have been forbidden from talking to people in the Courthouse (not unusual in highly competitive areas), one of the Landmen will more than likely be able to help you.
You also might want to check with your neighbors that have leased - all of them dealt with a Landman at some point and will know who you can call.
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas