Some of y'all here at Flares are, IIRC, medically and/or chemically knowledgeable. I've put some research effort into this and it has not helped advance my understanding much. (Having said that, however, I've just recently discovered and begun exploring the ConsumerLabs site.)
The bottom line is that I'd be grateful for the help of those who know a bit about the use and, in particular, procurement of "dietary supplements".
As far as I can tell there is no good way to judge the quality of sources (or, for that matter, the particular chemical combos that various providers choose) for such items as CoQ10 (see also), Omega-3 (and/or EPA), glucosamine and chondroitin (our vet says he takes Cosequin) and EDTA.
The use of these has been discussed with the appropriate medical professionals and is either advised (or not specifically discouraged) and is not contraindicated in any detectable way. They are all pretty low on the bad side-effects scale (i.e, relatively harmless if not beneficial).
What I cannot get advice (beyond what I can discover for myself which is not satisfactory) about is from where to procure reasonable quality at reasonable price. These sorts of supplements are available most anywhere for a wide range of prices and there is no method I am aware of for determining price/performance.