Almost any vitamin/supplement store has a variety of arthritis supplements. I don't know the best affiliate program in that market off hand, but I know Swanson's vitamins has some of the best prices, a good rep and an affiliate program. Programs like that aren't going to payout as high as MarketHealth or a CPA network rebill would. Other thing you can do is to try searching OfferVault to see what CPA networks might have an arthritis offer.