Model Truck & Fire Apparatus has alot of nice stuff, no light bars but plenty of other things, tools, nozzles, truck cabs, a pumper body, wheels, sirens and some lights. The website is very basic and there is no online catalog but it is worth ordering a paper catalog, it is probably 40 pages.
http://www.mtfaco.com/
Another one is Spalding Trading and Shipping, this is mainly truck parts but they have some stuff you can use for emergency vehicles and has the best slam locks (door latches) I've found, they are photo etch and you get 20 (10 round, 10 square) for $5 or so.
http://www.stsmodeltrucks.com/
There were a couple others but their websites have been down for awhile and I don't know if they are still out there or not. You also might check ebay, there are a few selling lightbars there from time to time.