not sure where to source one or cost, but I read a really neat account of agent who made into the Hardrock 100 and lived in Texas iirc. He put lots of miles in training, but he purchased a mask that you can adjust the amount of O2 you're getting and used it on stair climber
I want to say he finished in the top 20 which is really saying something in that race
might not be germane to your elk hunt, but a neat story nonetheless:D
