I've asked a similar question on the main board - but mine is related to younger dogs!
My guy is on Synovi3 which has all the good stuff in it - glucosamine, chondroitin, MSM. antioxidants, etc. It's pricey, but online supply companies have it a bit cheaper than my vet sold it to me for.
Vet did say that to get the full benefits from glucosamine you need the MSM or some sulphur compound - helps with delivery.
Have used Glyco Flex with younger guys - seems to work fine.
If you go to the refuge forum, DOC E sells canine suppliments. I personally buy 100% pure forms of glucosamine, MSM, chondtritin, and vitamin E & C from health food stores and supplimend my dog's that way, and it's alot cheaper than "special"