you will never get a high end fish as an adoption. high end fish, such as anything wc, hold their value steadily no matter how big they get. If someone had a gem they likely wont get rid of it and if they do its likely they already have people lined up to buy it. The one exception maybe being red tail catfish which I highly would not recommend as they will eat any other special fish you want to put with them.
That being said, you can still get an oddball or beat up fish that you could revive to full health and then you have a very strong connection with your pet. Sometimes the most boring fish will have the best personalities
Out of curiosity, why are you looking to adopt reputable fish when you could just go out and buy them.