https://aliexpress.com/
https://auctions.yahoo.co.jp

aliexpress is chinese, merchandise condition is not guarantee. but sometimes can found really, really cheap one than ebay.
yahoo auction is japanese, condition is really good.(because of restrict member evaluate system) but system is hard for foreign people. there's no international postage, so you should use your own country's proxy site.

Thrift stores: Rarer and rarer, as most stores just junk computer parts that don't come boxed, or throw them on ShopGoodwill, ebay, or similar.
Garage/Yard sales: Rare, as people tend not to decide to put twenty-five+ year old tech out on a neighborhood sale on a whim.
Estate sales: Possibly common, as collectors and appreciators of old tech are catching their deaths more and more rapidly.
Craigslist: Depends where you live. Nobody sells old tech within hundreds of miles of me, unless they want reseller prices for Commodore 16s or something.
Ebay: It's there, if the asking prices and shipping don't make you sick.
Alltronics: Never used it myself, so I have no personal testimony.
TXMicro: Hit or miss, as they don't have nearly as much old tech as they used to, but they do frequently sell cheap office computers.