Cola is a really hard flavor to nail. Pretty much any cola you buy in stores that isn't Pepsi or Coke is almost certainly going to taste like those cola-bottle gummy candies rather than the flavor you associate with Coke. There are a lot of different spices and citrus oils in there, and getting the proportion just right is really tough (it doesn't help that Coca-Cola is literally the only company in the country allowed to use coca leaf as a flavoring. They have a special permit from the FDA to import it.)
I almost always prefer drinking my own home-made soda to store-bought stuff, but I've given up completely on trying to formulate a recipe for a cola that tastes even remotely like people think cola should.
If you're deadset on making cola with a SodaStream, I'd go to a commercial restaurant distributor and get your hands on a box of real Coke or Diet Coke syrup.
Not at all. I brew my own cola at home using the OpenCola [1] recipe -- and speaking as a person who is very picky about colas, I find it delicious, easily on par with store-bought Coke -- which you could easily produce on a large scale.
The recipe is very close to the original Coca-Cola recipe; the Coca-Cola Company now presumably uses synthetic flavours in place of the various oils, which means the homebrew version actually tastes much more fresh and natural compared to store-bought Coke.
(I will admit Homebrew Coke lacks a little bit of the "bite" of Coca-Cola, which might come from the coca leaf.)
I use a SodaStream-style home carbonator, but it's not great. Store-bought soda water is much better.
It's not hard at all. Coca-Cola themselves won't sell to you, but that's just because they don't have any interest in selling single units to individuals. I've heard that some Sam's Club stores sell it, and I've also heard of people having success by convincing a local restaurant to just order them an extra box from their distributor.
I almost always prefer drinking my own home-made soda to store-bought stuff, but I've given up completely on trying to formulate a recipe for a cola that tastes even remotely like people think cola should.
If you're deadset on making cola with a SodaStream, I'd go to a commercial restaurant distributor and get your hands on a box of real Coke or Diet Coke syrup.