Citric Acid/Ascorbic Acid

nopot2pin

You will find it in home brew shops.
My homebrew shop stocks it, but its not readily available on the shelf, and you have to ask for it, because of the drug assocaition.
Or look at online home brew shops. But their delivery can be quite high.

baby_fuzz

http://shop.waiyeehong.com/index.php?main_page=product_info&products_id=2208&currency=GBP48p for 100g is quite good. try your local chinese or asian cash & carry.

nanokitten

Captain Starlight, it is the acidity that matters not the chemical formular.
The best alternative (which is safe, easy to get hold of etc and doesn't smell like a chip shop) is tartaric acid - cream of tarter and is availabe in the baking section of supermarkets.
you could also try baking powder in place of both the citric acid and bicarb (as baking powder is a mix of bicarb and tartartic acid normally).
Alternatively you should be able to get citric acid in brewing shops, a quick google brings up plenty of suppliers, including healthfood, brewing and indian food shops here