Under certain circumstances, a grant through the ERASMUS program of the European Commission is possible. For more information (on this matter), please contact the study abroad office at your home university.
For possible funding as part of a university partnership, you can find out here whether your home university is a partner university of Mainz and who to contact in Mainz.
Also, you should look for a person to contact within your field of studies. A list of departments at the JGU with contact addresses can be found here.