700x18 is rather unusual. Tubes, however, are not sized as specifically as tires. You may get a tube labeled 18-25 or 25-32 or some other range. So I would guess that anyone who rides 700x23 tires would have a spare tube you could use. It may look a little large, but you should be able to make it work. Likewise your 700x18 tubes would probably work fine in 700x23 tires. Also not a bad idea to carry some patches in case you ever run out of tubes.