Spring Valley Triple Strength Glucosamine/Chondroitin Joint Health Dietary Supplement, 40ct: Vitamins : Walmart.com
(I'm not recommending this particular brand; just the ingredients. I would think that whatever brand you can get is probably fine.)
You should also go to the doctor and find out what is causing it. Sometimes I get severe pain from muscle spasms. It is possible that if you are sitting a bit off center either while riding or while not riding, you could be making things worse.