Joint Supplement Help?

I have a 15 year old OTTB gelding who has been having issues with his stifles, and when the vet came out and did flexions on him, he said that his front legs had a real severe case of arthritis. They recommended 2 options, injections or supplements. I am going to try supplements first, because according to my friends, it is a lot cheaper and more reliable (but I'm not sure, so if you guys could give me some info on injections, that would be extremely helpful). But I am unsure of which supplement to try. If I could get some recommendations from you guys, you would help me so much. Thank you :)