Here are some places to buy gear (these people cater to cavers ... imagine that!).
Attitudes are different "across the pond". Take a look at this amazing site from the UK. These guys are serious about mine exploring.
Visit this site for a lot of fun and interesting information: California Mine Explorers
Dezdan's site has been steadily shrinking, and now is all but gone.
The Nevada Mine explorers were about the most technical mine explorers that were publicizing their adventures on the web. Their site now seems to be gone.
The Desert Drifter site has changed and shrank and now is all but gone.
The Ghost Town Explorers site has some nice images of some nice young folks running around in the Carlyle Mine in the Dale Mining district, near 29 palms, California.
Take a look at McRocks. The field trip reports have lots of interesting stories and photos. The ones in Arizona and California (By John Cornish and Keith Wentz) discuss interesting locations, some of which are familiar to me. They are rather careful to not tell quite everything about how to get to some of the places they have visited, which is fine by me given how eager various authorities are to find and seal up old mines these days.
Tom's Old Mine Info / email@example.com