All ways to get new slaves in current version, not counting unique chars:
Guilds: Only common races.
Slave Market: Mostly common races with occasional rare.
Exotic Trader(every other week): Rare and Monster races. (no orcs/beastkin)
Bandit Den/Fort Beaten: 25% to get person of random race (anything goes) otherwise common local races.
Bandit Den/Fort Prisoner: Same as above just 10% chance, or 15%?
Goblin event in goblin den.
Fairy event in grove.
Tribal elf ambush event in jungle.
Notably you can only get orcs/beastkin races from the Bandit Den/Fort. They cannot generate anywhere else in 0.6.0b