Ele viveu de sua herança nos três anos anteriores a Azkaban, pois estava muito ocupado lutando com a Ordem.
O tio Alphard de Sirius deixou-lhe muito dinheiro no final da sua estadia em Hogwarts. Isso foi o suficiente para ele comprar uma casa, e mais tarde uma Firebolt, com o restante do dinheiro ainda somando uma porcentagem significativa da fortuna dos Oleiros.
“Where did you go?” asked Harry, staring at him.
“Your dad’s place,” said Sirius. “Your grandparents were really good about it; they sort of adopted me as a second son. Yeah, I camped out at your dad’s during the school holidays, and then when I was seventeen I got a place of my own, my Uncle Alphard had left me a decent bit of gold — he’s been wiped off here too, that’s probably why — anyway, after that I looked after myself. I was always welcome at Mr. and Mrs. Potter’s for Sunday lunch, though.” Harry Potter and the Order of the Phoenix - Chapter 6
Imediatamente após Hogwarts, Sirius se juntou à renegada Ordem da Fênix de Dumbledore para lutar contra Voldemort, e não teria tido tempo para nenhum trabalho importante durante os três anos antes de ir para Azkaban.
By the time the four friends left school, Lord Voldemort’s ascendancy was almost complete. True resistance to him was concentrated in the underground organisation called the Order of the Phoenix, which all four young men joined.
Pottermore - Remus Lupin