I think it's about twenty miles from Rainbow River to Crystal Springs. Rainbow River is a worthy trip. Back in the seventies, I did the entire river from the springs to the Withlacootchee River several times hanging under a boat. (You know you're running out of Rainbow River when the water gets warm and murky and you notice Alligators watching you)
Back then, there was an industrial gas supply that had "breathing air" in Dunnellon. Maybe check with the local Fire Departments to see who fills their SCBA tanks. Or even KP Hole. They should know.
We were there this past March. I did a couple of drift dives out of KP Hole on the Water Taxi. It was ok but I was shocked at the regulations. "You must wear the wrist band or get thrown out. If you step one foot out of the water, you're trespassing", etc. etc . We're heading back again the end of this year. We'll be staying at the Rainbow River State Park across from KP Hole and I'll be using my kayak to drift dive from.