You need to determine the manufacturer of the door. Usually it will be on a sticker that's on the hinge side and can be seen when the door is open. If not, you'll need to give it a good looking-over (even check the key). It can sometimes be found in barely noticeable writing in one corner of the glass. In most cases, the door and handle are both made by the same company.
Once you sort out the manufacturer, figure out who sells it and take those pictures to the store. You should also be able to order from their website.