If I needed a set of shower curtain rings and I was seeking to save money, here is what I would not do -- personally. I would not seek out used ones of any kind.
Shower curtain rings wear out -- that is the bottom line. (Well, most everything wears out, but used shower curtain rings just do not seem like all that pleasant an idea to me.) Metal ones rust and plastic ones get icky. (Yes, I said "icky". Don't you just love "share time" with Regi B.? Don't answer that!)
With that info "disclosed", here is what I would do.
Assuming shopping online for your new shower curtain rings is not your plan, I would simply go to my local Goodwill Thrift Store.
To be clear, I am not sending you there for something used. New items come into Goodwill, too. Occasionally, people may donate shower rings they have not used and are not going to use.
New or used, I hope your "new to you" shower curtain rings make you sing (in the shower).