In older clothes there are often three labels:
Designer
Retailer
Fabric mill or other
I have searches set for favorite mid century modern designers; but you can pick any decade that appeals.
for example, when I decided I wanted Norman Foster aluminum chairs (not the navy chair), my search ran for 2+ years before I was able to purchase 12 chairs for 250 each from Florida including shipping
the pickings are slim for what I want now, but again, it’s a matter of waiting. Often an item will not be labeled dead stock but NWT or otherwise be pristine. If an item is not dead stock, I prefer if there were underarm shields, that can be removed, to guard against excessive wear.
I prefer search by designer (Jacques feith, Donald brooks, Adel Simpson, Norman Norell; but will occasionally run a search by store (Ultimo, charivari, Linda dresner, Wilkes bashford, didier ludot, Anna lowe; bonwit teller). or, location, like British Hong Kong. or design element, such as, peplum. I have a continual search for Catherine regher, a canadian designer, bc I used to own the most magic dress ever (40 USD with custom label from Bergdorfs) but I loaned it to my cousin for her wedding after party (and it was left on the floor to be WASHED, probably with third world coconut soap). Urgh. Needless to say, I’ve never loaned my cousin anything else.
As you browse vintage searches, you will come up with other names, either suggested by the search engine or from whatever seller you happen to be perusing. There was once a woman from Iceland selling a warehouse filled with dresses on Etsy.
note: for vintage RTW, it is essential that you know your measurements; the silhouette that works for your body type. Also, factor in cost of cleaning, refurbishing etc. even dead stock may have linings that have given out (due to the weight or age of the garment etc). For example, my norman Norell coat is one of my favorite pieces, but the lining was not the best quality. It cost the same amount as the purchase price of the cost to have my tailor replace the lining in a very luxurious silk twill.
for fur or leather, be especially careful of condition. IMO it is almost impossible or very costly to rehab a dried pelt. Balding calf hair is a loss. I learned this the hard way when I bought a vintage lizard trench coat (not sure if it was Gucci, but it had a certain disco 70s flair that Tom Ford tried to resurrect). It could not be saved. i spent about 700 USD before giving up. The cost of refurbishment can exceed the cost of the item.
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one of my favorite pieces is a wool dress by Donald brooks from a random ebay seller for 57 USD. Such amazing wool fabric is impossible to find nowadays. The inverted pleat and pockets align perfectly
deals like that balance out more expensive vintage finds
also, one last note: pants are hard to fit. It’s possible to size up, but depending on the fabric, there may be marks. Sizing down does not always work. i suspect
@cowgirlsboots knows much more about tailoring vintage clothing than I will know in a lifetime, so perhaps she will chime in

. If you start buying vintage, go slowly. There is a learning curve and mistakes will be made.