[OutOfHere]

You can form a simple CO2 scrubber by buying colorized bulk soda lime, putting it in a HEPA filter bag, and running air through it, but it's very expensive. It'll probably cost $100 a day. Also, you'll still be on the hook for maintaining good ventilation so as to remove indoor bioeffluent VOCs.

Better systems are being researched.

[BizarroLand]

You can also buy an at home oxygen concentrator for around $125 usd and try it. Iirc it uses atomic sieves that filter out anything larger than oxygen and then releases the oxygen, so it can run for a very long time without replacement.

It's also fun to see how it affects things on fire. The weak little jet of air from the one I bought can turn an lit incense stick into a pillar of fire.

There are likely health downsides to them for otherwise healthy people so I wouldn't suggest using them frequently for breathing purposes, but they can help with short term aerobic output and exercise if you are training at the edge of your physical capability.