The municipalities around here advertise them in one of the local papers. The information is also available online, but the websites are always a few days behind the curve.
Like Sparky says, watch out for requirements for a certain amount of MBE, WBE, local residents, local Subs, etc. or you can get burned going for the job.
Even worse is forgetting about those things and then actually getting the job...then you can end up with a negative profit very easily.
They're also typically very slow to pay so figure that into your quote.
Good luck!!