Some larger cities will have their own labs. Some will farm the work out to other labs. Most city departments will have a ride along program. You can ride with an officer or detective for a shift and find out what the job is really like. Just call your local department for more information.
Every small town has a police force.
They will also have a detective or two, but those guys don't retire too often and competition is pretty severe. There will be a few support positions, but, obviously, the smaller the locality, the smaller the police budget, the smaller the police force. There are plenty of private agencies out there. Some are very good, very high profile and very well known. Most of these are staffed by retired federal investigators and support personnel.