Only one official source: Directly from Rogue Amoeba via their website.
No one gives away a $39 license key out of kindness. The websites hosting "keygens" or "patchers" inject malware into your Mac. Recent examples include:
Rogue Amoeba has been making Mac audio software since 2002. Their apps—Audio Hijack, Loopback, Piezo, and SoundSource—are industry gold standards. When you pay for a SoundSource license key, you are funding:
A cracked SoundSource license key is an illusion. Even if it activates once, a future macOS update will destroy it. Then you will be back to square one—only this time with a potentially compromised system.
