What's wrong with going to a welding shop? Cranston Welding on Park Ave sells CO2 fills for beverage service to homebrewers.

Let me also put in a plug for my friends at Blackstone Valley Brewing Supply in Woonsocket. They do CO2 tank exchanges. Not open today their days are WED-SUN but they are the best shop in the region if you also happen to be looking for a brewing ingredient supplier too.

https://www.blackstonevalleybrewing.com/