What scale? The free "Brigand caves" supplement for Stonehell is a big cave lair with like 20 bandits which I got a lot of use out of. ACKS Lairs & Encounters has a simple 200 brigand lair which is more like a Roman army camp (and like 100 other lairs).
I don't have a bandit lair but I have a free booklet on bandit NPCs that talks a little about the culture of rural criminals. Wrote it up to be compatible with Swords & Wizardry and my Majestic Fantasy RPG.