There’s not a lot going on for the Sennheiser HD480 replacement foam earpads. Your choices seem to be limited to a few countries. UK have a few merchants selling them, and of course there’s always thomann.de, but they only deliver to European countries.
UK
http://www.dv247.com/invt/33561/
Europe
http://www.thomann.de/se/sennheiser_hd480_ohrpolster_02.htm
US
According to the code numbers given by Sennheiser (37893) and given the link below, these foam earpads should fit the HD480 headphones.
http://www.bhphotovideo.com/c/product/352705-REG/Sennheiser_H37893_H_37893_Replacement_Earpads.html
Official Sennheiser Page
http://www.sennheiser.com/sennheiser/icm_spa.nsf/root/service_spare_parts
