street/ illegal source:
1)Theft (we can assume this is direct theft by criminal)
2)Drug Dealer/ Off street (we can assume this means the criminal bought it from a non-organized vendor of illegal goods. We don't know original source though.)
3)Fence/ Black Market (we can assume this means the criminal bought it from a reseller of illegal goods for whom that is his sole business rather than a one-off side thing like #2)
Those are my theories anyway, but it doesn't help us pinpoint any source of the gun, really. If Bob bought a gun legally at a dealer, traded it for crack, but his crack dealer got robbed and the thief sold it to a black market guy who gave it to his cousin, it really help much of anything to know his cousin bought it from a black market dealer.