Buy Refills? good god no! Unless you have more money than sense...
Yes, its just yeast and sugar (how ripped off did you feel when you opened the can and it was full of sugar and a little vial of yeast....)
There are plenty of recipes for the refills around, do a search on here for "diy co2", Or here is a link to a very comprehensive guide http/www.qsl.net/w2wdx/aquaria/diyco2.html you can just use the recipe from that.
One good thing about that red sea system though, I have found the little diffuser to be really effective, and it takes up so much less room than some other diffusers!