I suggest
Volg
The nearest from Affoltern is Regensdorf or Wallisellen.
Maybe you want to call them first, if they have any at all.
Filialen im Kartenausschnitt
Volg Regensdorf 1
Dorfstrasse 140
8105 Regensdorf 1
Tel. 044 840 31 66
Fax 044 840 32 61
Volg Wallisellen
Riedenerstrasse 81
8304 Wallisellen
Tel. 044 830 33 08
Fax 044 830 33 08
Local farmers are another option - they also sell them.
Google terms like Chemineeholz or Brennholz.