Here's a tube, it fits 3.5" x 10" tires and is made explicitly for Lambretta scooters.
Made with a slightly offset air valve so the valve will not interfere with your fork link.
Not the most exciting thing in the world but if you need them, then you need them.
*NOT FOR VESPA USE* The stem offset will interfere with your rim if you try and fit them on a Vespa so don't do it.
Includes a fancy valve cap with a valve core remover.