You can for example use https://www.ebay-kleinanzeigen.de/, https://jasprtrades.com/, the local recycling company, https://www.freecycle.org/, the projekteverteiler Leipzig and other sharing websites
good luck! 
Things that are needed:
- building material:
- plastering, Putz
- electrical cables, Stromkabel
- waterpipes (steel, 1/2 and 3/4 inch), Wasserleitungen (Stahl, 1/2 und 3/4 Zoll)
- ladders
- living/office material:
- matresses,
- bedding plus sheets and cushions,
- kitchen (probably two) plus everything needed for it like pots, pans, cutlery, herbs and spices,
- wardrobes, shelves
- tables, chairs
- lamps and bulbs
...