Most motor factors will sell the boots for ball joints, but best to take an old one with you to get the correct size, as they aren't always listed on their computers or in catalogues.
The way of fitting the new ones will become obvious when you get down & look at them, but if you are not familiar/experienced with suspension repairs, best leave it to someone who knows !
These are the sort of thing you need
http://www.bailcast.com/ball-joint-dust-covers/