I put quite a few hours into research this a couple of years ago, cross referencing various sources trying to find some common threads in the hope of sorting the bullshit marketing from the truth.
(From memory, so might be off the mark a little but the gist should be close enough)
The biggest problem with todays diet is we only eat the nice palatable parts of the animal now. A LOT of what we need to build tendon/ligament/joint/connective tissue strength is in the bits we throw away (bone/marrow/sinew/connective tissue.. nasty stringy shit) (Home made stock would probably be a good way to recover this?)
I found a few products which 'claim' to supply the nutrients(?) and after researching these 2 products stood out, of those 2 'Animal Flex' got the most consistent good reviews and personal recommendation. Sorry, i dont remember the other product.
Bone broth is supposed to be 'the' best!
http://paleoleap.com/eat-this-bone-broth/