Back issues?

I've recently gotten it in my head to try and collect all the STH back issues. Being that I was rather young when they came out originally (2nd or 3rd grade) I didn't quite take as good of care of them as I should have. I realize that this is going to be a rather monumental (and expensive) project but I'd really like to get the whole run (specials and mini-series included). Does anybody have any suggestions on where to look other than eBay or MidTown comics online?

I do my best to grab whatever back issues my local comic shop has, and have started picking up the most recent issues so I have a pretty formidable collection started considering I've only just started but any help is always appreciated!

tl:dr trying to amass the full run of STH, where do you find your back issues?