Well you can look in the Textures category of the downloads on this website:
https://tf2maps.net/downloads/categories/textures.22/
If you aren't finding ones in hammer, then search "sign" with the filter "tf" in the texture browser, and it shows like 90% or so of all the default overlays tf2 has.
Also, you can download content packs like Frontline, Construction, Japan, etc, and they usually have a lot of signs and other overlays.
Whether you download them from the site or you get them from a content pack, just make sure you pack them into your map!