I have a Drinkmate CO2 cannister - do you ?exchange these at this location

Yes, they sell them in-store and even on line they will exchange them by sending you a return label with your purchase.

You can exchange canisters but not tanks.

I was under the impression you simply buy another canister and you're supposed to properly dispose of the old one?

Yes

Can't exchange in store. You send it back to be refilled