Pretty much any non-aristocratic civilian figure from such companies as Wargames Foundry and many others would do. Though you might have to convert them a little and put suitable weapons in their hands.
Of course you could also use spare figures from Lord Curr's Incorrigibles and the Scotland Yard box sets. I'd keep a close eye on North Star as I'm pretty certain that they will be releasing more figures before too long.
I'm sure the chaps here will come along with a host of other choices as well.