Synechocystis sp. PCC 6803 was isolated from a lake near Oakland, California, USA.
in the case of PCC 7120 / ATCC 27893, Rippka R, et al 's paper states that the source of its isolation is unknown, the ATCC website mentions that this strain was derived from an existing strain.
PCC 6803
Article Stanier RY, Kunisawa R, Mandel M, Cohen-Bazire G.. Purificat...
PCC 7120
Article Generic Assignments, Strain Histories and Properties of Pure...