It comes after reports this week that Doherty has been injecting heroin in his cell.
Doherty, 29, is said to be planning to smuggle out a "kite" - an unauthorized note - to Moss after landing himself in debt one week into his sentence.
It shows how desperate the situation is - Doherty and Moss split up last year and Moss is now with The Kills frontman Jamie Hince.
The insider said: "She may seem the most unlikely person to call on but when they split, that didn't stop Pete contacting Kate.
"Pete hasn't got any money until his prison allowance comes through, so in the meantime he's had to borrow and owe from his inmates for things such as tobacco and daily toiletries. He is still sleeping on his pee soaked mattress.