What to use instead of John Dory fish?
Looks like what you read was correct.  From Fine Seafood Co here is a description of the John Dory fish. (As many in the US will not be familiar.)


  John Dory
  Often named St Peter’s fish (St Pierre in France, Janitore in Spain), as he is alleged to be the origin of the distinctive dark ‘thumbprint’ (or spot) on its side as a thank you for some help with his tax! The English name John Dory arrived from the French ‘jaune doré’ meaning ‘golden yellow’ – a good description for this unusual looking fish, which varies in size from smaller 230-450g fish up to 2kg. Because of the large head (like Monkfish, this accounts for half its weight), John Dory is best filleted, but beware the low yield (around 35%) and some sharp nasty spines, which require extra care when filleting. The flesh is creamy-white, with a dense texture similar to Dover Sole, which holds up well during cooking. Ideal pan-fried or grilled John Dory works well with Mediterranean flavours, salsas, and peppery sauces. It may be an expensive fish with a low yield – but it’s worth it!


(Emphasis mine.)

Per my research, Dover Sole seems to be the first choice to use as a substitute for John Dory. Another that I found mentioned quite often was Turbot. But that seems to be even more expensive than John Dory.

Others listed as possible substitutes on Cookipedia are: Bass, Grouper, Snapper, Emperor, and Halibut.

Here's a link to the Guide on Fine Seafood Co with descriptions of many other fish. (No endorsement intended - just good information.)