Do they sell an oxygen tank/mask you can wear while you ski? I feel like people would buy the crap out of that.
Sure, you can buy those single use canisters like people have mentioned:
https://smile.amazon.com/Boost-Oxygen-Natural-22oz-Cans/dp/B00U1P3B1Y/ref=sr_1_1_a_it
And you can buy refillable oxygen cylinders and regulators (I'd use a cannula stead of a mask) but they're going to be heavy because to carry enough O2 for a day they have to be highly pressurized and able to withstand carrying around. And you have to get them refilled at a medical supplier. A rather spendy proposition, which is why people don't buy them so much for recreation.
I work at a ski area where the base is 10.3k ft and the top is over 12k ft and one thing I notice is that I start breathing deeply, like a runner, automatically just going up steps or walking up hill. Some people not familiar with altitude don't do that and they get a little freaked out when they find themselves gasping for air. I tell folks "breathe early, breathe often" and lots of times they are more comfortable when they realize that it's just that there isn't as much air and of course they're going to have to breathe more deeply.
And when folks come into the patrol aid room with headaches, dizziness, shortness of breath, and nausea, they get their vitals and history taken and a quick exam to
rule out other issues, which is very important. If there is no other suspected cause, they get O2, usually via nasal cannula, and they have to read and sign a page with information about altitude sickness that essentially tells them that though they are likely to feel a lot better after getting O2 going to lower altitude is the cure.
IME as a patroller, fitness isn't really a factor for altitude issues, but people that don't have a basic level of fitness may find themselves exhausted
and sick from altitude, which is an uncomfortable experience. Nutrition, rest, hydration, and time at altitude seem to have an effect.