Many Walmart stores also carry recharged SodaStream CO2 bottles - you take in your empty bottle and they give you another for about $15. Some online reviewers have had trouble getting the refilled cartridges at their preferred Walmart stores, so you'll probably want to call ahead to make sure your Walmart has them in stock before carrying in your empty cartridge. Certain Bed Bath & Beyond stores also carry refilled CO2 cartridges for around $15 with trade-in.
The SodaStream kit comes with one bottle for making the carbonated water. You can buy additional bottles for around $15 to $20 a pair, but there's really no need. I recommend picking up some lidded plastic bottles at your local dollar store. You can pour your homemade soda into these bottles for drinking and/or transporting.
That way, you don't store soda in the plastic bottle used for making carbonated water. You can keep it filled with chilled water in the fridge, ready for the next batch of soda.
You won't want to store the soda for very long anyway. Just like store-bought soda, your home brew will lose its fizz before too long and won't taste as good the next day. You should count on making only what you can drink within a few hours.
The SodaStream doesn't need batteries or A/C power, so you can take it to the park, beach or pool along with a cooler of ice water for making your favorite drinks on the fly. At 5.98" by 11.89" by 17.72", it doesn't take up much space, whether on your counter or in a tote bag headed for outings.
With a SodaStream at your disposal, you can always be prepared for unexpected guests or excursions.