Also check Walmart. According to the website, I see a couple of Walmarts by where I live stock it (in Bedford Park and Forest Park.) Both have citric acid in stock under the name Ball Bulk Citric Acid, so I assume found in the canning section. Picking a more northerly ZIP code, I see the Walmart in Niles and the one near Cicero and Diversey also have it in stock. Check their website.
Which reminds me, maybe Target might also have it, since they carry canning stuff -- yup, the one by my house has it, so check their website, as well.