The Wrightson SWAMP THING stories in issues 1-10 are reprinted in multiple places, and easy to find. (Although the remainder of the 24
SWAMP THING issues have not yet ever been reprinted, despite some good stories by Michelinie and Redondo. )
The MAN-THING stories are far less often reprinted, particularly the
SAVAGE TALES 1 story (May 1971).
The MAN-THING origin story was re-told (I think far better) in FEAR 10 by Conway with Chaykin/Morrow art (Oct 1972).
And the
FEAR series ran in 10-19 (1972-1973), before it moved to its own series in
MAN THING 1-22 (1974-1975).
Like the origin stories, both regular series began and were cancelled at almost the exact same time!