Well, as someone who buys too many bikes I've got a few "usual" sources.
The main is a local classified site. I honestly find a lot of deals there that are within an hour or two drive from the house. This is also my primary sell source as I don't like shipping and possible problems with it. I want the buyer to see things with there own eyes and be totally happy. Even if it means a loss for me. There's a few other Paceliners that sell stuff on KSL as well as here. The mint Pinarello I bought from a KSL ad was also listed here for a long time before I pulled the trigger.
https://classifieds.ksl.com/search/Cycling
Then there's here and the C&V classifieds over on Bikeforums.net. I also picked up that special Lemond from a Wisconsin seller on the Retrobike classifieds.
A few have come from facebook. Like the Paris Brest Paris bike I bought directly from Bob Fourney. I'm still leery of Facebook purchases though and need to really be able to confirm things before I buy something long distance off of there.
And of course Ebay occassionally. Usually something a bit obscure, or classifed weirdly or something. Like the NOS Trek Y77 Y-Foil frameset I found a few years ago listed under vintage steel or something like that. I was the only bidder and got it at around 1/2 the price of other recent NOS sales. Or the lovely Losa built Krapf frame I nabbed. Not listed as a Losa build and I actually watched this frameset sit on there for 5 years until the price got to where I got it. I've nailed several good deals on NOS vintage steel on Ebay with just digging deep and deep into the sales, especially sellers in like Holland and Belgium.
And.... try hitting up distributors to see what old stock they could still have sitting around in their warehouses. I got a NOS mid 90's top end Giordana XL Super frameset from Gita around 2015 just by emailing them to see what they had. At the time they still have several of the Giordana frames available and a fair amount of other NOS Italian bikes they had distributed over the years.
For the heck of it. Here's all the current rides and the last column has the source.