I think the best ideas have already been given, but I'll list off what I think would work best.
1: Schools. I've gotten the most cheap/free stuff from schools. The trick is talking to the right person. Sometimes the IT department/computer teacher is in charge, but I've found that the head of maintenance is usually in charge. Sometimes you have to follow the trail to a town or county employee responsible for all kinds of surplus, from computers to desks to school buses and police cars. Some of them can give "worthless" stuff away, or sell it for a dollar to just show they got something for it. Others may have to auction the stuff off. Which leads us to...
2: Local Municipal Auctions: If it's a silent auction bid high enough to outbid anyone trying to buy for scrap value...which wouldn't be that high anyway. I've gotten good deals from town auctions. The ones around here are always sealed bid, never ben to a live auction.
3: eBay, craigslist, etc: Keep looking online, especially for anything close to you. I've never posted a want ad on craigslist or another similar local site, but have heard of people getting results. Sure won't hurt.
4: Find the ones people have hiding in their closets, basements, attics, etc. Find ways to talk about people's computing history. I've found the ones that come from individuals who have owned them since new are most likely to be in great shape with lots of software disks and accessories. With any luck they're happy to give it all to someone who'll actually use it and appreciate it.
5: Always keep your eyes open. Might be obvious, but it's worth mentioning. Curbs on garbage day, anyplace you go... When visiting a friend's family business two of their first three computers they ever had were on the floor in one office. I got two Lisa 2s with their keyboards and mice, software, some old magazines (I think this is where I got my fist issue Macworld), manuals (at least one still in shrink wrap), and an original keyboard box and accessories box. I traded them a Power Mac that was then about 5 years old in trade, which I had gotten in school surplus for $1. I've told this story before but it's my best "they're out there" story.
As one last piece of inspiration, earlier this year a I was given a functional SE/30 from a friend who had it in his basement. I hope you find some cool old Macs!