UCLA has many of his memos from the original series in the Roddenberry papers. The one from TWOK is in the Nick Meyer papers at the University of Iowa. I haven't located any other Trek memos by the company as of yet, though.
(Although de Forest himself certainly wrote or co-wrote the ones for TOS, Joan Pearce apparently was the one most responsible for the TWOK memo, according to other documentation. When de Forest's company went bankrupt, the company she moved onto, Joan Pearce Research Associates, took over the franchise).