Were they all residents of Texas? If so, then you can get certified copies from the county their estates were probated in and record those in the deed records of the Texas counties they owned minerals. You will need the Application for Probate, Order Admitting Probate, Letters Testamentary, Will and any Codicils, and the Order Completing Probate (specific titles of the aforementioned documents may be slightly different). You can also file a certified copy of the deed in each county as well.