First thing is checking they're rated for your weight. If they are, I think the problem is not a loss of pressure because I would've gone soft very fast in a couple up-downs. So imo (not a specialist in pneumatic things, but I'm pretty good with machines in general) it must be the valve you move with the lever not closing properly. I have a couple chairs at work that don't have spring-loaded levers and you have to push down them for the chair to stay at the chosen height, so try pushing the lever a bit. If it's an outside problem—something bent or badly adjusted, dirt or something that stops the movement of the mechanism— then you might be able to fix it, but if it's an inside problem with the valve itself, it's worn out or gets stuck, then I'm afraid you'll have to replace the whole piston assembly.

To answer your questions, as I said I don't have that much experience with pneumatic parts, but parts in general are never too standard nor very custom, that is they usually are standardised but there are so many standards and specifications and variations that yours may be hard to find. Look for references/codes printed on the cylinder, the specifications you'll need to find another one would be diameter(s), length(s), displacement, pressure/force, there could be also different kind of fastening types and ways to actuate the valve.

How hard it's going to be, I can't tell, but it's most likely possible to do.