I hate when this happens, I just changed all mine last week.
Basically you need to know the make of the door.
Going to your local lumber yard may yield better results.
See if there is a tag on the door usually at the hinge side, or on the hinges themselves sometimes there is a manufacturer.
Most of the time you wont find anything, but bringing a picture of the door can help at the local guys.
Plan b is to get a sweep which goes over the entire bottom which needs to be screwed on.
Hope this helps. Good luck.