The best one just might be Elderly Instruments:

The Famous Old Time Music Company is run by a friend who’s a great banjo player:

Once in a while you can find used classics at:

This is a great all-purpose site with musicians’ classifieds and a ton of other information:

Then there will be your local and national music stores and guitar shops where sometimes a used instrument will come in on trade; your local Craig’s list, and any other local classifieds you might have.

I wouldn’t buy a vintage guitar — or any guitar, really, but certainly not an expensive one — sight unseen on the Internet, but you can check out the following sites:
(Elderly Instruments, Lansing MI) (Mandolin Brothers, NY, NY) (Gruhn Guitars, Nashville, TN) (House of Musical Traditions, Takoma Park, MD)

All of these dealers have actual brick-n-mortar music shops where you can go and look at and try out the instruments in person before you buy. I would highly recommend doing that before purchasing anything.

Many can be found listed on the web search “vintage guitars”
One of my favorites is:www.gruhn.com.
Good Luck