Hi Teecee,
Here's the only ones I can find that are not included in ORBX or FSAddon's libraries. I believe these come with default FSX.
Main library location is: Scenery\Global\scenery\buildings.bgl
Objects are: Gen_FireTower01 and Gen_FireTower02
Also, I know ORBX has already placed quite a few fire towers in their regions so I recommend you make sure you're not duplicating these first.
Brad