There aren't any objects that look like signs, but a couple of tilesets have signposts in them. I have time to kill, so I'll make a list:
1 - Tutorial World (a classic wooden signpost)
43 - Mertynn Forest (a strung cloth with writing)
59 - Mertynn Cavern (like 43, but there's more choices)
81 - Wwods Tileset (several signposts with designs on them)
142 - Simple Nature tileset (look familiar? The exact same sign is in tileset 1)
154 - Color tileset (two colorful signposts)
155 - Nowhere (a steel arrow-shaped sign pointing right)
There's one more (something like "Tutorial World, Gradienty edition"), but I forgot its number. It doesn't really matter, though, because it has the same sign as tilesets 1 and 142.
But remember, a Sign doesn't have to actually look like a sign. You can place whatever graphic you want there so long as it stands out to the player and looks like something that could have text written on it or that could talk. Plenty of tilesets have sign-worthy stuff in them (for instance, the open book on a stand in Tileset 20, the papers in Tileset 72, etc.), so look for stuff in whatever tilesets you're using to build a screen. Improvise.
If you're building a screen and neither of your tilesets has a good sign, look through the object banks for a person or animal that doesn't wander around and put a Sign on it to make it talk. This is actually a lot more common than actual signs.