Since I'm not on facebook, my only options are eBay, Craigslist, and thrift stores.
Of those three, thrift stores have provided the best bargains.
I'll buy parts and supplies on eBay, but I've never bought a vacuum that way. Generally, the cost of shipping makes it impractical.
I've gotten a few from Craigslist. They don't show up often. But, when they do, it's sometimes a good deal. Right now, in fact, someone has a beautiful green Eureka 2091 D, in what looks like really good condition. But, they're firm on $80 for it. That's still cheap compared to an eBay price, plus shipping. But, compared to thrift store prices, it kind of hurts. If it was an absolute dream find, or like one from my childhood or something, I might go for it. But, under the circumstances, I'll probably have to pass.