Local law-enforcement agencies are buying cellphone-tracking equipment that is cheaper and smaller than earlier systems, according to documents reviewed by The Wall Street Journal, but it isn’t always clear whether court orders are needed to use the devices.

The systems, which go by trade names such as “Jugular” and “Wolfhound,” are handheld and sometimes come with antennas so small they can be attached to clothing, according to public documents. The gadgets cost only a few thousand dollars each—far less than more sophisticated...