Prof,
I am guessing that you may want these for Mordheim/Witchtown.
I have managed to put together a Thieves warband using old 80's Citadel figures, scouring ebay mostly, utilising 'C; series Thieves and Rangers and the odd adventurer-type. They will eventually all be painted (hopefully!) as if is 'night', either in monochrome or a blue 'night-lighting' effect that is often used in films.
('Harry Carter and the 14 Thieves' will soon see action, I hope!)
If it not important what range the figures come from there are a lot of options using pre-slotta stuff and even some of the earlier ranges that were aimed at AD&D players, such as Ral Partha, Grenadier and early Citadel. More recently, Reaper produce quite a few, using their search option should make it easier.
Is it only human thieves you are after, or a mix of races?