Two choices. Rent from Linde or AirLiquide or buy a small cylinder. Have a look here http://www.laboutiquedusoudeur.com/bouteille-gaz-sans-abonnement/bouteille-argon-co2 ; They have bottles for sale; Not cheap,but might be more economical in the long run. From memory, I pay around 80 euro a year rent for my Mison 8 bottle and about the same for a refill.
I own a few in date bottles, but no one would touch them for a refil. If friendly with a bar owner, you might be able to get pub gas, CO2, which I used to do. Then they mixed something else in for more head and that didn't agree with welding.