Get yourself a countertop carbonator like a Sodastream or Drinkmate.
I have a Drinkmate, and they have a CO2 cylinder exchange program where you can get a heavy discount on more cylinders by returning the empty ones beforehand (prepaid shipping label included with cylinder packs.)
Other than the initial cost of the machine and any additional cylinders you want (I have a total of five, so I can install the last one and ship back four empties for a coupon, but that's probably a little overkill for one person), the ongoing cost is reasonable. I make 1-2L of soda water pretty much every day and I get a little over a month out of each cylinder. Works out to less than $15 a month for me.