Interesting mikvaot and links about mikveh (mostly historical, some current)
Mikveh Mayim, R' Yirmiyahu Katz (Hebrew, 3 volumes)
Mikvah Israel of Borough Park (58th St.) is built on a ma'ayan (spring)
The Chabad mikveh in Tenafly is as well
History of mikvaot in New Haven, CT (interesting to the Jews of New Haven!)
Includes 18th-century engraving of the mikveh in use!
Mt. Sinai Congregation in Cheyenne, WY has a mikveh fed by an underground river (operational as of April 2023; verify kashrus before use)