bath fizzies/citric acid

[mezzanine27.livejournal.com]

i'm interested in making bath fizzies. all the recipes call for citric acid, and i thought i had read somewhere that you can use crushed vitamin c tablets. but then i read the ingredients on a bottle of vitamin c and it didn't contain any citric acid! so my question is, is it possible to use vitamin c? if not, where can i buy citric acid? thanks in advance