Titleist doesn't have a used club site. There are used club dealers like 2nd Swing with both an internal web site and eBay store fronts. You have to be careful with eBay. Even authentic SC's that have been modified with a 3rd party cannot be sent to the SC shop for refurbishment/customization. GolfWRX has a swap meet section. I've played with both LH and RH putters to get a feel so my 3 came used. The one constant is nothing feels like SC. I'm now waiting for LH Phantom 7.5 to be available to finally make the commitment to a new SC.

I know you can find them at places like 2nd swing and golf galaxy.

There are also services by Scotty where you can send in your old Cameron putter for refinishing and customization.

If you're looking for older/more uncommon models, there are some club swap Facebook and Reddit groups, I know there are a couple Facebook buy/sell groups specific to Cameron putters. Those may give you better leads than Ebay or local classified ads.

eBay is a good place to find older models.