Sirius and Gringotts

[Robert Jones]

No: HPFGUIDX 95704

How did Sirius get money out of Gringotts to pay for Harry's 
Firebolt in POA?  He could send Crookshanks with a note to the broom 
shop, but how did he get the money in the first place?  I mean he 
couldn't just stroll into the bank and identify himself.  And even 
if he sent a note with his name to authorize the withdrawal and a 
place to send the money, they would see his signature and call the 
Hit Wizards. 

Bobby