Sorry, Ron - I never had need of that much refrigerant before! Have you talked to some of your local mechanics to find out where they get theirs? That's where I'd start...
I am not local to you but we have a Carlisle auto air here I think they have locations in other area. I am not sure about refilling them I think they are 1 time use but you can purchase new cylinders.
I did a google search and found these guys in Tampa Auto Air-Conditioning Parts, Custom Manufacturer - Florida, FL
have you tried them?