I am a pharmacist by profession. Thats a lot of money for citric acid. It usually is about 2-5 bucks a pound. If you want for cooling system flush use nonpharmaceutical grade stuff. Food grade citric acid is just what you are looking for.
I did a web search for you and found following online supplier.
http://www.bestdeal.org/Merchant2/merchant.mv?Screen=PROD&Store_Code=Essential&Product_Code=CITRIC-1%23
I do not have any experience with them so call them up before you buy. I do not have any experience with flushing the cooling system using citric acid either.
Good Luck
MVK