Several from old grannies and grandpas selling stuff on Craigslist, a couple from the MIT Swapfest and other flea markets, some from the FredMiranda and MFLenses Buy&Sell forums.
A few from eBay, some good deals pop up every now and then if you're patient for a few months and set up notifications based on keywords rather than thinking "I need this lens right now".
Sometimes if you see an auction on eBay with a high starting price and no bids, it's possible they have been re-listing the item for a long time, and you can message the seller and say you'll buy it right now at a lower price if they turn on that option; many would be happy to be "done with it" and sold. Sometimes if the seller is local you can convince them to cancel the listing and do an off-eBay, in-person sale for cash at a discount (eBay charges a lot of seller fees).
If you bid on an actual eBay auction, place your bid with the highest price you are actually willing to pay and ONLY in the last 10 seconds, or preferably last 3 seconds. By doing that you will either lose OR win but get it at just slightly higher than the next highest bidder's max price and significantly less than your stated price, while leaving that second bidder with no time to re-think and re-bid. By doing the single last-minute bid, you effectively turn the auction into a sealed-bid, second-price (Vickrey) auction for yourself, which is most efficient for you in a game theoretic sense.
KEH has very reasonable prices on most old lenses. Not cheap, but very reasonable.
Also, if you're willing to take apart a lens with a slight amount of haze/fungus, sanitize and clean the hell out of it (not for faint of heart), and put it back together, you can get some very, very steep discounts. Or if the lens has a couple of scratches, you can get steep steep discounts due to loss of antique value, you don't need to take anything apart, and it will have zero noticeable impact on wide aperture images.
A few from eBay, some good deals pop up every now and then if you're patient for a few months and set up notifications based on keywords rather than thinking "I need this lens right now".
Sometimes if you see an auction on eBay with a high starting price and no bids, it's possible they have been re-listing the item for a long time, and you can message the seller and say you'll buy it right now at a lower price if they turn on that option; many would be happy to be "done with it" and sold. Sometimes if the seller is local you can convince them to cancel the listing and do an off-eBay, in-person sale for cash at a discount (eBay charges a lot of seller fees).
If you bid on an actual eBay auction, place your bid with the highest price you are actually willing to pay and ONLY in the last 10 seconds, or preferably last 3 seconds. By doing that you will either lose OR win but get it at just slightly higher than the next highest bidder's max price and significantly less than your stated price, while leaving that second bidder with no time to re-think and re-bid. By doing the single last-minute bid, you effectively turn the auction into a sealed-bid, second-price (Vickrey) auction for yourself, which is most efficient for you in a game theoretic sense.
KEH has very reasonable prices on most old lenses. Not cheap, but very reasonable.
Also, if you're willing to take apart a lens with a slight amount of haze/fungus, sanitize and clean the hell out of it (not for faint of heart), and put it back together, you can get some very, very steep discounts. Or if the lens has a couple of scratches, you can get steep steep discounts due to loss of antique value, you don't need to take anything apart, and it will have zero noticeable impact on wide aperture images.