Robert Indiana kept a series of illustrated journals during the late 1950s and 1960s, in which he discusses the development of his work as well as his daily life on Coenties Slip. 

This journal page covers February 26, 1959, and February 26, 1960. The first entry, for February 26, 1959, includes a sketch of Source II in a vertical orientation. Notes indicate the different colors to be used in the painting (which differ from the colors of the finished work), as well as its medium (oil on Homasote) and dimensions (59 7/8 by 47 7/8 inches). Indiana also records an evening call from John Hoppe (to prepare for the AT&T show, Bell Telephone Hour on NBC-TV, in which Indiana participated on March 4, 1959), and that he visited a few galleries.

In his entry for February 26, 1960, Indiana describes a visit to the showroom of Lilian and Herbert Ross.