To be able to complete your doctorate in the set time frame, it is of utmost importance that you have a reliable financial source for your doctoral project. Some of the scholarships offered are only granted if you apply from your home university, as in the case of the DAAD (German Academic Exchange Service) scholarships.
Following is a list of links with possible scholarships for you to check:
Apart from receiving a scholarship through a foundation, often it is possible to be employed as a project staff at the chair where you have applied to do your doctorate. Please ask your advisor to give you information on this matter.