The breathing air cylinders available from industrial gas suppliers are often Grade D as that is the OSHA required standard for work at atmospheric pressures. SCUBA is typically Grade E at a minimum.
You can get Grade D & E air and pre-blended Nitrox from Airgas or Linde, but you're not going to like the price - typically more than $0.50 / cu ft, and you'd want a booster to get it out of the cylinder and into your dive cylinder. Tack on the cylinder rental, hazmat fees, etc. and you'll quickly be at $1.00 a cubic foot.