What are good sources for specific back issues of publications like Analog?
I've already looked in the normal places (Amazon, Google, etc.) but I suspect I'm going to need to search some kind of specialty source but I don't know of any. I'm not looking for discount sources and am even willing to pay a slight premium if that's what it takes.
I'm looking for general answers because that will be of value to others and I'm more than willing to do the rest of the leg/mouse work. The details if anyone cares: I'm looking for a three part story that was published in late '08 and so far I've only found the 2nd of the three.