It's been my experience after many years in the Corvette restoration world that the best source of information and advice for which supplier may or may not have the best NOS/used/rebuilt/restored/reproduction parts is other enthusiasts, especially with the instant communication provided by internet discussion boards like this one, the Team Camaro forum at www.camaros.net
, etc. Many suppliers carry the same parts, some are better than others in terms of NOS parts, accuracy of reproduction parts, condition of used parts, accuracy of description of parts, etc. People have different experiences with different suppliers, some suppliers have good days and bad days, and no one single transaction conveys a consistent overall evaluation of a given supplier.
I've kept my own notes over the years on suppliers, and update them regularly based on what I hear from others, factoring in the individual circumstances of each good/bad experience and the skill level and expectations of others; someone new to the hobby with limited skills and tools might consider a given used part a POS, while another more experienced restorer might consider the same part an excellent candidate for restoration. There can be a significant difference in perception between those who consider "restoration" as buying a part and installing it out of a box and those who will actually "restore" a part before installing it.
That's the primary reason that neither the NCRS nor Bloomington Gold, in the Corvette world, endorse, certify, or recommend any supplier or parts manufacturer, or maintain any "list" of "approved" parts or suppliers. All that matters is what the part appears to be on the judging field after restoration, and to what degree it appears to meet or deviate from the published judging standard - there are lots of different ways to meet that standard, depending on the skills and resources of the restorer.
"Restored" means different things to different people, and that definition is completely different for "show-n-shine" judging vs. originality judging; the goals are different, and so are the paths to follow to get there.