I think first you want to consider whether you are after a second-hand or "old-stock" historical mask, namely one dating from 1975 or earlier, probably an oval model with a rubber skirt and a stainless-steel band, or whether you would like to purchase a new "classic" mask made by a modern manufacturer in the traditional way. Remember, unless the vintage mask is for display only, that it's a very personal item of gear that has to fit, and seal against, the wearer's face, and three and a half decades or more will have taken their toll on the mask's skirt which may have hardened or even perished during the intervening years unless it has been well looked after!
If you plan to go ahead and buy an historical mask, I agree that eBay or Vintage Scuba Supply is the way to go. The Vintage Double Hose folks will also have some good advice and expertise to share with you. If, however, you're prepared to explore other vintage-style mask options, then here's my two-cents' worth:
1.
Cressi sub Pinocchio

Cressi still makes one rubber-skirted mask and the Pinocchio has a cult status, the more so because the original design dates from 1952. It's available here in the UK from Amazon.co.uk, here's the URL:
Cressi Pinocchio - Original Rubber Diving & Snorkeling Mask: Amazon.co.uk: Sports & Leisure
2.
Oceanways Classic Pacific mask

Comes with a stainless steel band. Available here:
Discount Divers: Oceanways Classic Pacific Mask
3.
Classic Oval Rubber Mask

Steel band, tempered glass, classic design and inexpensive too.
Classic Oval Rubber Mask: Scuba Gear and Diving Equipment
Classic rubber-skirted, stainless-steel banded oval masks are also made by two French manufacturers, Beuchat and Sommap. If you're looking for coloured mask skirts, your search will be much narrower and more arduous and you'll probably have to forgo the steel band. My favourite modern classic mask is an Ixtapa, made by Escualo Sports of Mexico:

I had to wait for over a year before I could track down somebody online prepared to sell me one!
I hope I've provided you with a few options. If you want to look further into what is available in the international modern classic diving mask marketplace, just let me know.