where do i buy citric acid?
there should be some under chemicals and metals here
You might try your local health food store. You can also find it in some types of photographic stop baths.
Your local homebrew store and/or lumber yard will carry it, though you can use orange juice just as well.
I've seen citric acid sold at a middle eastern grocery store/restaurant.
If you need a whole pound, you can order it at the local (NOT big chain) pharmacy. They may want to know what you're using it for.
health food shops
It is probably sold in supermarkets as lemon salt or some other close name.
Or you could try a place that sells wine/beer making supplies, for example, http://www.midwestsupplies.com/