This depends on two things - where you live and the make/model of your door.
There's often a plate or sticker somewhere on the inside of the door that tells make and model - google around using what you know and you'll find people who specialise in spare parts. - And people who offer to repair them too.
http://www.doorfit.co.uk/pages/buy_products.php?menu_id=10&sales=1&source=google
Good luck...
Peccavi, March 2009
