Most food co-ops will have a bulk food section. You should be able to locate it in most of them. Brands are immaterial. Citric acid is citric acid, all commercially produced.
1-5lbs seems like a lot. You may have a hard time finding that much. I follow these instructions: http://www.coffeeco.com.au/articles/august2002.html and it calls for 10g per descale. Unless you are planning on buying a lifetime's supply 1-5lbs is overkill. I got 50g from my pharmacist for free, good for 5 descales. I seem to remember Orphan Espresso selling citric acid on their website for a reasonable price.
Any home beer making or wine making shop should have it. Also, look in a Kosher market or the Kosher section for "sour salt" but verify that it is just citric acid in it.
I searched "citric acid" on amazon.com; there's product sizes from 4 ounces to 5 pounds. Orphan Espresso's Citric Acid Espresso Machine Descaler Crystals page lists various sizes at competitive prices (including an attractively priced "just clean it" kit that bundles citric acid with Joe Glo). For what it's worth, just about any supermarket in our area stocks it in small 4 ounce jars labeled "sour salt". As Randy noted, beer/wine making supply stores will have it in bulk since it's used for sanitizing equipment.
I didn't have any luck finding some locally, the home brew stores had something else that was like it but not the same. It's cheap on Ebay and they offer bulk amounts.