There's a bug in the current version of the game that blocks Mudbrick from offering Maggie's room in the upgrades before you finish Mariam's quest with a slave choker. It's fixed in the upcoming v0.8.
As for the current version, you can finish Mariam's quest and then buy the upgrade. She says the slavers should be around Thula's outskirts. You'll find their base in the dangerous outskirts zone to the left of Thula's map entrance.