This has been a consideration of mine this year. Prior to getting into beer, I was considering getting a soda stream. (I think I still will) and my reasoning is this:
For a full keg/CO2 tank setup, that would be great, but I would want my carbonated items chilled, requiring the addition of a kegerator/fridge. This puts me upwards of $500 just for the kegerator (premade or self made), co2 tank and co2, lines, used corny kegs(x3).
While it's still my aim, I'll be getting the soda stream because I can carbonate small bottles and chill them for personal use. Mostly carbonated water, but I think it would be good for a co2 test as well.
I bottle carb my beer, cider and soon mead, and plan to continue this process since if I'm aging them anyways, I might as well. But, there are times when I have about 28 ounces left which I plan to toss into one of these bottles, carb and have a taste test that resembles the final product that day. (Usually beer, maybe a sweet cider. Unlikely mead since that does better aged.)
(Plus, I have an espresso machine so I have a few dozen syrup flavors so I go through carbonated water pretty quickly.)
Other options include building your own carbonator using a co2 tank, lines gague and modifying a cap to be able to pressurize a standard 2 litre pop bottle. There's instructions online, but I don't feel confident enough to do that yet. (That would probably cost $130 or so).
As for Sodastream, depending on your location, you can often find stores that will do the replacements. Williams-Sonoma does this.
Btw, you may have wanted this in the soda making forum, or the kegging forum as opposed to the mead forum.