/r/OldSchoolCool had an image of a black cop and a white agent stopping a fake lumber truck during Prohibition in LA (1926). I found some info on the history of black cops in LA, but I was wanting to know if there are more sources.
Not very. The LAPD hired its' first black officer in the 1880s, and had their first black lieutenant and watch commander in 1940. But the department was segregated and only had about sixty black officers around the time of the Watts Riots in 1965, out of a force of roughly 3,000.
source: the LAPD has a "history of the LAPD" page.