Hotel rooms in different categories can be found through the various online portals such as hotel.de, hrs.com, and Airbnb. Many hotels in the city center of Mainz are close to the event locations.
The youth hostel in Mainz is located next to the Rhine and a park. Buses 62 and 63 will take you there from the main station. Only few beds are available, so you will have to be quick, if you would like to book one. Beds can be booked via telephone (06131/85332) or email. Let them know that you belong to the Fulbright Alumni group. Information about the youth hostel is available here.
A map of public transport in Mainz is available here.