Agree, 700x18C is a *very* small tire. It is also a very unusual size, I don't think I have seen that size around any store. Normally the are either 19, 21, 23 etc The more universal size used would be the 700x23c. But if you like the 18 then go for it.

And tubes I normally use are the Vittoria Ultralite 700C and they fit in tires ranging from 19c to 23c.