I've had a lot of fun creating my own gang called the Firebirds. I uaed some Funky Skull figs converted with head swaps and some from other sources.
I've found that Westwind Productions Arthurian Pict heads make good heads for using in conversions because they have long hair but are clean shaven.
The Assault Group does a nice collection of three packs of rioters in their ultra modern range which would be very suitable for Street Wars and they were sculpted by the main sculptor of the Funky Skull figs, Nick Collier.