Even if you have an actual CO2 tank, if it's just a bit too old you have to have the hydro and everything done on it in order to get anyone to fill it.
So we know you're going to have to do that at a bare minimum. And then the valve on a scuba tank is going to be wrong for connecting to a CO2 regulator (or at least I would assume that to be true, I don't actually know for sure). So you're going to have to buy a new valve and have someone replace it for you. I could be wrong, but I'm guessing that's not something you can really do properly in your garage with a big-ass pair of pliers.
It's a cryin' shame, but I'd bet money that by the time you get done converting a scuba tank to be "legal" for CO2, you'll spend at least double what it would cost to just buy a shiny, new aluminum CO2 tank.
Just one more example of the **** government sticking their nose in everybody's business and trying to extract that last penny out of us whenever we choose to exercise the freedom that they have oh so "generously" granted us.
OFF TOPIC RANT: All those extra fees, surcharges, and regulations on a CO2 fill are imposed by the DOT. Why can't I get an exemption from those if I walk to the shop with my tank to get it filled?