Not to worry. The tube with just enough air in it to fill it out will measure 700C X 18mm. It will expand to fit your 700C X 23mm tires. I'm not certain how far over the printed size you can go with a tube but it is pretty far.
Most tube retailers don't give a size in mm for the width of the tube anymore. They just list them as 700C Small, 700C Medium, and 700C Large. 700C X 18mm is considered to be small, and your 700C X 23mm tires take a 700C Small tube.
Not to worry. The tube with just enough air in it to fill it out will measure 700C X 18mm. It will expand to fit your 700C X 23mm tires. I'm not certain how far over the printed size you can go with a tube but it is pretty far.
Most tube retailers don't give a size in mm for the width of the tube anymore. They just list them as 700C Small, 700C Medium, and 700C Large. 700C X 18mm is considered to be small, and your 700C X 23mm tires take a 700C Small tube.