to add to what greg said ...
if you are near a university
see if the library has a special collection ---->>> print collection.
head up to boston, go to tufts and to the library's special collections area
they have portfolios of callahan and others to look at ... just ask.
if you are making a day of it, go to haaavaaad, make an apt the day before
go to the fogg and sackler and ask to see photography,
the stuff is there for looking ... if you end up in prov ri, go to the risd museum,
they probs have oodles of callahan and siskind's work, seeing they taught there ...
have fun !