When the sign is given by the candidate to a citizen to place on the citizen's property it becomes property of the citizen. If the citizen gives you permission to take the sign, it's yours. A drive through your neghiborhood the late evening of election day, or a day or two after will get you a nice supply with permission.
In our area, such signs are not permitted on public property and are fair game for removal. They are removed and trashed by road crews. Lots of them pop up on public property in the early morning of election day. I get most of mine from roadway medians the night of election day. We go through 30 or 40 signs a year on our hunting club's rifle range.
But don't go on someone's property and take a sign without permission.