A New/Old Cache of RAH Material - unz.org
If you haven't seen this yet, unz.org has a huge online library of materials all the way back to the 1840s including periodicals and articles, books and films.
I tried a few Heinlein searches and turned up a number of interesting items including:
"Science Fiction Hawks and Doves: Whose Future Will You Buy?" from Ramparts
magazine, Feb. 1972:http://www.unz.org/Pub/Ramparts-1972feb-00025
A critical review of Time Enough for Love
by John J. Pierce in Reason
magazine, Septmeber 1973http://www.unz.org/Pub/Reason-1973sep-00024
"They" in the original publication in Unknown
, April 1941.http://www.unz.org/Pub/Unknown-1941apr-00084
There is a lot of other material that I haven't had a chance to look into yet--you might give it a try (even for subjects other than Heinlein!) at unz.org.