....There are many manufactures, do a search for amateur radio (ham) antenna mounts (base)... base being a building verses a mobile mount for vehicles....there are also D.I.Y plans available on the net as well....you won't need to weld anything...just have the ability and tools to cut and drill 1/8" metal......there are also some sites out there that can help you figure wind loads and whether or not you might want to use 1/4" metal depending on the stress being put on the structure due to height of the mount itself(higher the mount, less weight that can be safely supported vs. a shorter mount being able to support more weight) The weight of the siren and related hardware figures into this as well.
Word to the wise: don't build anything so tall as that later on you can't do simple repairs if needed safely....been there and done that.
