Like ISAF says...use the pressures on the info tag on the vehicle, not the "max pressure" or "max load at xxx psi" marking on the tires. And note that some buses have different air pressure specs for front and rear axle tires.
If you need air while out and about, and you're near a truck stop, you can find high-pressure air hoses and proper inflation chucks near the semi fuel pumps. The little inflation stations out front, for the passenger cars, which take quarters often won't go above 90 psi or so. The air hoses near the semi pumps have enough pressure to air you up and they don't require quarters. And remember that the air pressure specs for your tire are for a "cold" tire. As you drive and the tire gets hot, the air expands and the pressure will increase...so you don't want to let air out of a hot tire to get to your specified pressure, because when the tire cools the pressure could be too low.