What is the best joint supplement for the money spent for 18 year-old and 20 year-old that are still working well! Something that will help injections last longer etc? Thanks

Needle!!  Adequan, polyglycan, legend, etc. Feed throughs are very expensive and very little bioavailability . My 20 year old mare gets pokyglycan monthly ! 

I use MVP Exceed 6 Way, I have vet proof its worked on multiple horses of mine. There was a significant chnage in joint fluid consistency in horses the vet had injected for years, and it deteriorated again when I pulled them off once. They're all on it now, it's extended the life of my injections. You can also go the inectable route, which is where I would go if I didn't have the Exceed. Whatever you decide, do your research. Anything that doesn't list all ingredients, including the amounts in a guaranteed analysis is a scam. And anythign the doesn't have the NASC seal is a huge red flag, there's a reason they don't have it. If you're going a feed through route as well, I wouldn't pick anything without at least 100mg of HA in it and nothing under 10,000mg MSM 

Adequan 

I like Myristol Pro

Adequan