Maz Kanata helps her get it
In the Canon animated micro-series "Star Wars Forces of Destiny", Leia is shown defeating Boushh and presumably stealing his armour
Maz: Frozen in carbonite?
Leia: Oh! Chewie said you can help us. We wouldn't ask if Han weren't in a tight spot.
Leia: So what's the plan?
Maz: Patience. The solution will present itself.
Leia: Ugh! We've got a problem!
Maz: No That's the solution!
Maz: You need a disguise. And a bounty hunter is perfect!
Leia: Oh, you've got to be kidding me!
Maz: I'd say this armor is just your size.
Season 2 Episode 6 - Bounty Hunted
While the tale in Legends is similar, Boushh getting into a struggle with the Black Sun crime syndicate and ending up dead, it wasn't Leia that directly stole his armour. In fact, it is Xizor's Human Replica droid, Guri which ends up killing him, for demanding extra payment.
Guri then provides Leia with the Armour for her to infiltrate Coruscant and meet Xizor. Organa kept the armour and once again used it to infiltrate Jabba's palace. [Citations coming]