I'm not sure about specific counties, but I know that many Freedmen's Bureau records are held at the National Archives in Washington, DC. Check the NARA website for their holdings. Before I visited National Archives, it was helpful to read some history books about African Americans in Texas, including:

Crouch, Barry A., THE FREEDMAN'S BUREAU AND BLACK TEXANS. University of Texas Press, Austin, 1992.

Smallwood, James M., TIME OF HOPE, TIME OF DESPAIR: BLACK TEXANS DURING RECONSTRUCTION. Kennikat Press, Port Washington, New York, 1981.