Jay:

Offut in Nebraska as well as Andrews AFB also have chambers. It is very
informative and fun. One of the guys in our group volunteered to take his
mask off at 25,000 feet. Even with the instructors yelling at him, he was
unable to gang load his regulator and replace his mask, and it only took of
few seconds. It was a pretty dramatic demonstration of the time of useful
consciousness at 250. The people who did best without their masks on at
18,000 were me (terribly out of shape), and a woman flight surgeon who chain
smoked. Go figure.

I suggest contacting the aerospace physiology office at these bases for a
civilian ride, since they are the ones who manage the chambers.

Good luck.