I dont know a link but I fully recommend knee pads (dirt bike or down hill mountain bike ones are the best that cover the shin too, as it help to keep your leg straight and stops the pad from moving)
a helmet, not many people use one but I always do and I know a lot of professionals do to. You think you will never hit your head but you do. plus it makes you feel a lot safer when going for that new trick. protec or any bmx helmet seems to be best.
Wrist guards. Always good to have when you start and fall a lot. I always have a pair with me for when trying new things but dont always wear them as you cant use your hands with them on. (when you start wear them, after you know how to fall, optional)
Where to get them from I think 'Jason' has covered it.
Joe