First off we kinda need to some information about your group, like levels and classes.
Depending on why both your players party and the NPCs are "prowling subterranean tunnels and cave trails" you could start off with some RP. Having the players encount the NPCs just as they are making camp. The slavers see an opportunity, so invite the players to share their campfire and some of their tasty meal. Perhaps they have a very strong, bitter dwarven stout that like to laced with drow poison. And/or they wait till second or third watch for to a delicious suprise round. The PCs should have plently of opportunities to sense motive and spot signs these NPC arn't not on the up and up.
If that's not for you, or you know your player won't fall for the bait, ambush is your best bet.
1) Rogues built into the Justiciar from Complete Warrior [pgs 47-48] (just reflavor it to match a slaver ideal and remove that pesky lawful requirement). You might want to dip Fighter for 1 or 2 levels for feats, BAB and weapon proficiency. The Bring 'em Back Alive will allow you to use a normal weapon as subdual and still deal sneak attack(as subdual) + nonlethal strike damage. NOTE: Using Ninjas in place of (some) Rogues to gain access to use poison for Drow Poison.
2) You'll need some sort of distracting, tanky npcs. Trippers will be your best option, but grapplers are also useful. Prone/Grappled victims are easier to hit for subdual damage as well as easier to initiate/join a grapple. While not a net, the Mancatcher from Complete Warrior [pg 154 & 157], is a reach weapon that can, as a free action, start a grapple check against any medium or small sized creature you hit with it without provoking an AoO. In addition, once a creature is grappled, they can attempt a trip attack, without the initial attack being made. (I've seen this used effectively, especially when all the melee used reach/ranged weapons to attack the trapped opponent.) Remember that bolas and especially nets (if you miss or they escape, -4 to attacks until you spend two rounds folding the damned thing) are effectively one time use items per combat and they both require a feat to use. In addition, only bolas deal damage.
3) Bards built into Mindbender from Complete Arcane [pgs 54-56] to complete the group. They can buff, heal and cast charm person. Pick up the Practiced Spellcaster feat to up said buff durations once you get into Mindbender. If you can get the PCs to share a camp with the NPCs, Telepathy is a great way to awaken the NPCs without alerting the players. Spells like Grease, Blindness/Deafness, Silence, Charm Moster, Haste, and Invisibility Sphere provide a great deal of control over the battle/preping for ambush. If you can get the Use Magic Device skill up to a point that you can hit a DC 20 check often, picking up a Wand of Stone Shape {Divine} should allow you to divide and conquer.
4) Wizard built into Elemental Savant from Complete Arcane [pgs 32-34]. Pick up the Nonleathal Substitution (+1 to spell level) and have the option to turn all your damaging spell nonleathal. Can you say subdual fireball damage? (Honestly this one is more of a joke, but I figured I'd leave it here anyways.)
Clever slavers will not want to look/act the part of slavers when out searching for new pery. An easy guise (or even a possible side job) for them to claim is being bounty hunters or monster catchers. That will help explain the abundance of non-leathal weaponry, extra chains, manacles, etc. (This is assuming you want to take the RP approach.) If you decide the slavers have been working their tunnels for a long while, they could have easy place serveral covered pit traps along key point. They will want to try to seperate the PCs, confine them to a limited space, and keep them locked down until they entire party has been sudued. While you did mention wanting a small group, you may want to consider having a number of pairs of Rogue/Tripper equal to 3/4 of the party, a single Bard and if you really wanna take the party down a wizard.
Well I wish you the best of luck, and I hope your players enjoy what is to come.
