Try Carparts4Less although looking through the cross reference on the Contitech website it looks like the year 2000 aircooled 126's have a different belt that the older a/c models, the correct belt according to the Contitech reference is part number AVX10x800.
These belts are also fitted on the 1990 Fiat Panda 900 models if you are using Carparts4Less there is a 14% off code for this weekend WEEKEND14 there is also a freepost option on checkout.