Sella names his source: the doctor's name is Enrico Lazzaro, already investigated for doping at the 2001 Giro. Apparently the drugs were sourced from a hospital in Serbia.
It also turns out the the Italian police had been investigating Sella for some time: they intercepted a Gerolsteiner rider's dad with a well known coach who was smuggling a syringe hidden in a toothpaste tube to his son at the Giro, with traces of Iutrelef (no, I do not know what it is, google it, but it's on the banned list). They then found 80 (yes, eighty) boxes of viagra at the coach's residence. Apparently the viagra was for, amongst others, a rider who is a good friend of Sella's. That got them on to him.