Aha - yes, it was a while ago - so long ago that I've started repainting them and reorganising them now that my paining skills are a bit better - there's a blog at
http://www.dakkadakka.com/dakkaforum/posts/list/346076.page The majority of the rank and file models are the metal 2ed Adeptus Arbites models - I managed to get a huge lot of them on eBay. They're quite expensive to buy nowadays, though.
All the special weapons troops are Necromunda Enforcers, which are still available from
GW in the Specialist Games area of the website. The Enforcers are a bit spindlier, but have much better weapon options - a box contains a proctor, cyber mastiff, heavy with stubber, and five arbites, plus sprues which have boltguns, shotguns, plasmagun, flamer, grenade launcher, power maul, supression shield and various pistols and grenades.
The Arbitor General and a few other models are from the Void 1.1 'Junkers' range, the leaders of which look very Arbites-y. These are also discontinued but Scotia Grendel still sell them in the
UK. This is also where I got all the extra riot shields, since all the plastics in the original Void game have shields.
The Repressor and Exorcist tanks are from Forge World, in the Witch Hunter section.
If you wanted to scratch-convert, then the closest models are
SM scouts. The plastic box handily comes with Shotguns, too! I'm planning to use some Scout Bikers as a base for Arbites bikers. You'd need to file the Aquila from the front of the armour, and convert some Arbites helmets. I've found that certain Warhammer Chaos Marauder helmets are quite good matches (there's a prototype on that blog above) and they're easy to get from Bits sites.
Hope that helps!