Hi, Evan --
If you're looking for research papers, you need to be looking in the
Journal of Cave and Karst Studies (JCKS), formerly the
NSS Bulletin. Although it's in the works, we don't yet have copies of either in the
Publications section of caves.org. Further, our search engine is not presently working properly.
However, the good news is that all of those publications
have been archived by the
Karst Information Portal (KIP)
http://www.karstportal.org/, and all are completely searchable.
As for the best elevations to search for caves, there's really no right answer -- they're everywhere. Perhaps the best way is to pore over topo maps, looking for intermittant streams. That indicates they go underground, possible via a humanly traversible opening. But while these can occur at any elevation, the watercourse generally winds up at the valley floor as a resurgence. Another feature to look for, either on maps or in the field, are sinkholes, which are evidence of collapsed cave passage. Maybe there's an open entrance to the passage there, or somewhere, or maybe you'll have to dig your own.
Since organized caving has been popular in the US for nearly 75 years (since adequate light was available), it's fair to say that all the obvious cave openings have likely been found, and new cave will require more scientific methods of discovery, and a lot of elbow grease.
Happy Hunting!
Alex