I don't think that physical maps existed for the 1850 census. They appear to have started in 1880 based on this document: http://www.archives.gov/research/microfilm/m1930.pdf
However, there are written descriptions of the enumeration districts on microfilm for the 1850 census which can be found in NARA Publication T1224.
"National Archives Microfilm Publication T1224, 146 rolls, provide written descriptions of these census data collection areas based upon selected civil division boundaries, streets, and other features."
I don't think that they have this microfilm at the Johnston County Heritage Center. You should probably check with the NC State Archives to see if they have it.