I found mine on ebay. Just make sure it's still within it's life span date (7 years I think???). If they are inspected within the listed life span (and pass..) they will extend the inspection life on them another 5 years or so. or something like that..
Some fire safety places sell referb. tanks as well. Or go to a local fire department and see if they are getting rid of any old tanks that are close to being out of date.