Make your own soda! Head down to your local homebrew shop and ask for a
soda making kit. You'll get a bottle of soda extract (comes in many
different flavors) instructions and some yeast. After you mix up the
extract, water and sugar (you can use real sugar, not corn syrup) you
add a little bit of yeast, seal the bottle and store in a cool, dark
place for a few days.

The yeast starts eating the sugar, producing CO2 which will pressurize
the bottle and carbonate your soda. It will add a little bit of alcohol
to the mix as well but only something like .1%

Of course you can also use them to condition (carbonate) beer too, but
the soda setup is much cheaper (~$5) if you're not already into
homebrewing.

Cheers,

Josh