I used to get CO2 canisters at Kroger or Best Buy. For the past 6 months Kroger's supply has been erratic and Best Buy has been non-existent.
I got a Soda Sense account and bought two additional bottles. They send them in a reusable box. When I have two empties I send them back. I really didn't need three bottles since the turn around time is a matter of a few days, but I didn't know that at the time.
The cost to exchange is $19 each bottle (about 2 bucks cheaper than Kroger). Sounded like a good deal, especially since the website said "Free Shipping" all over it. Turned out free shipping only applied to the initial purchase. So many people complained that they now ship exchange bottles free.
https://www.amazon.com/Soda-Sense-Cylinder-Cartridge-Refill/dp/B07WHHW2Y2