Ebay
I frequent one of the sellers, Karensbarn. I have bought several machines that were brand new but had been displays, or missing the original box, maybe some attachments, or manual. Karensbarn, also sells returns or damaged items, so you may have to dust it up or do a minor repair, but usually at 1/4 the original price.
Sometimes they sell lots, and you can get 3, 4, or more in the lot for one price. They also have flat rate shipping on all items for $8.95.
I got a Hoover canister that was a return. It was clogged with hair. Looked like they had cleaned a barber shop and pet salon with it. Cleaned it out, it's perfect.
I go a Panasonic MC-CG917 for $65. The original box was torn up, the manual was missing, everything else was complete and had never been used. No damage to the machine what-so-ever.
You just have to watch the pictures and read the descriptions to see if this is what you want to get into.
Some of the machines have gone for way over what they were worth. I think some bidders get hung up in the auction. Recently a Miele C-2 sold for $545. You can buy new in the box, warranted for less. Sharks sell all the time for $45. Dysons always command a price above what I am willing to pay. Bissells and Hoovers show up all the time for good prices.