Recommended Joint supplement?

I'd like to put my mare on some sort of joint supplement because I'm riding her so often and she sometimes seems a bit sore(other than that she is completely healthy). Does anyone recommend anything? I'm not looking for a super expensive supplement that needs to keep her from going lame. Just something that will give her a bit of a boost. I know some people who give their horses MSM but i'm not sure if it will work. If anyone knows of a good product thats not extremely expensive, it would be greatly appreciated!!!