I Have Patellofemoral Pain Syndrome In My Knee's And My

I have patellofemoral pain syndrome in my knee's and my knee's give out at random times and hurt badly and crack when I walk sometimes and I can feel my joints grinding every now and then and they don't sit right. Everytime I go to a doctor they can not figure out what is exactly wrong. I would like to get a new knee brace for my knee's. Any suggestions?