Radio Shack.
But at many RS stores, you have to ask for the PCB Etchant, not ferric chloride. I called nearly every RS in my area when I wanted to etch some blades and asked for ferric chloride. Most said "oh no, we don't carry those types of chemicals". Then, I actually got ahold of someone who said "Yeah, we have it, it's but we call it PCB Etchant". I called back to those other stores a day later and asked them if they had "PCB Etchant". "Yes, we have several bottles" was their reply. It's $10.49 for 16 oz.