When a voltage is quoted for a relay by a seller, it is usually the nominal operating voltage. The manufacturer in his spec. sheet will quote a series of voltages, the lowest will be the lowest voltage that will allow the relay to operate, the highest will be that at which the insulation on the coil starts to melt. The normal operating voltage will be somewhere in between.
When voltage is applied, current flows and the coil becomes a magnet which moves a lump of iron called the armature. This in turn operates a switch, or, in some cases, a bunch of switches. This switch works a circuit that can be totally independent of the one working the coil, so a completely different power supply can be used very simply. The restrictions are that the contacts have to be able to handle the current that the load will impose, and must also be able to switch, and switch off, the supply voltage without welding themselves.
Voltage reducing circuits that go down to RX voltage (look up BEC threads) are amazingly small and cheap because there is a huge market. Other voltages are not, they are for a very small market and availability goes down and price up to match.
Generally, an AAA cell will keep a radar motor going for days on end, and is lighter and cheaper than any voltage dropper. Just a manual switch, or have the battery easily accessible for removal after use and insertion before.
If you use LEDs for nav lights, current requirements are very low, and a PP3 battery does very well. If using 3volt incandescent bulbs, then, as Dave suggested, a pair of NiMH cells wired to give 2.4 volts will do the job. The lights will under-run which will prolong their life and will make them look more like navigation lights.