Most big cities have architectural salvage stores.

Junk shops sometimes have such things. Barn auctions too. Not everywhere has Restores, but I've seen a lot of old hardware there too.
I know there's also specialty companies online, but I haven't checked any of them out.

I search ebay until my eyes water and glaze over.
Casey

If it does not have to be actually old, but accurate reproductions, House of Antique Hardware and Vintage Hardware are two good sources. There are also several online sellers of genuinely old salvaged hardware.

Look no further...Robinson's is THE place to purchase incredible restored antique hardware. I made all my purchases for pulls from him when I re-did my kitchen 7 yrs ago . He has amazing stock and is always very well versed in his descriptions. You will spend hours looking but the reward is great and much more reliable than any other place you will ever look. Good Shopping !!

Here is a link that might be useful: antique hardware

We purchased kitchen pulls from House of Antique hardware and had some delivery issues with products being out of stock and I was never notified. The customer service rep I dealt with was rude.

I went to my local (45 min away) salvage shop and they sell reproduction hardware. They had the same exact pulls and knobs and I saved $100. They have a website and I believe most of their stock is online.

Historichouseparts.com