an easy place is a sporting goods store that caters to paintballers
they should both sell tanks and refill them.

an alternative is a privately owned tropical fish store not one of the
chains

Albert Owens wrote:

Just purchased about $400 worth of CO2 equipment (solenoid, bubble
counter, reactor, ph controller, etc.).

I thought it would be a simple phone call to get a CO2 cannister. The
vendor told me to go to the local fire station. They looked at me like
I was crazy. "What do you want a CO2 cannister for?" "Aquarium Plants"
I said. "I've never heard of that" was the reply I got. Anyway they
told me they couldn't help me.

I have tried local hardware stores. None of them offer it.

Tried looking online. Pressurized CO2 can't be shipped.

Did I just blow $400?