Question about joint supplement for horses.?

I have two horses that I keep on a good joint supplement. It costs about $100 a month. One has arthritis and the other has stifle issues. And the supplement makes a big difference for them. But it's awful expensive. I was wondering if anybody knew if I could make my own supplement for cheaper? I know MSM and Glucosamine is the two main ingredients and I can buy that for cheaper. Any ideas would be appreciated. Thanks!