I would say about 95% of my players use Co2, and most get them filled at Dick's, (they cost $3.99 per fill 20 ounce, and you get one free with four fills). Since I have so many tanks, I usually get my small air horns filled ($1.99 each for 9 ounces) and that uses up two of my fills, then fill up my 20's. I think I have about 6 or 7 tanks in hydro right now.
I got a compressed air tank (smaller 3K) for testing, but haven't used it much. I figured if I'm having issues in the cold I can use it instead. I can also fill it at home with a hand pump used to pump up high pressure pellet guns. So I never have to worry about being "out of air" when an idea for a test comes to mind.
Getting a compressed air tank filled around here past 3.5K is almost impossible (unless you go to a field with a high output compressor, and those are only open on the weekend)... But, getting Co2 is easy. (Northern Cincinnati area).