you have to show a certification card to get scuba tanks refilled. The low pressure port that powers the BC (bouyancy compensator) is 30psi. Releasing lots of air quickly (as in airing up several tires) may cause condensation within the tank, which shortens the time before needing service. Having a tank rolled and hydro'd is fairly expensive, as are the tanks themselves. Commercial co2, nitrogen, etc should be cheaper since your life doesn't depend on them being squeaky clean inside.
I've used my scuba tanks for "emergency" air for Years when I was doing classes most weekends...causing no damage. I mainly use two specific tanks for utilitarian purposes (air mattress, tires, boat tubes), and I save my good ones for breathing.