Goodwill is particularly bad about this. You can almost never find cool stuff in the stores anymore. It's all siphoned off for ShopGoodwill.
It was definitely one of my luckiest finds, but I think another part of it was that it was a pay by weight bin outlet. A St. Vincent De Paul in my case. Stuff isn't individually priced and you weigh your items on a scale when you check out and pay by the pound. It's usually very cheap because mine has a maximum of $5.00 per item so even heavy things like typewriters have been cheap.