I found myself in this situation last year when I decided to hook up my PS2 again. My OEM controllers were all lost a while back and all I had were a couple third-party ones.
In the end, eBay was the only real option I found. I paid too much for an unopened controller because I wanted to be sure I had at least one I could trust and they aren’t going to get any cheaper. I also took my chances on a couple “refurbished” ones that fortunately seem to work fine. And if they end up having issues, the parts that most often wear out are not hard to replace.