are you actually willing and able to budget in any more equipment? and if so, how much money?
its not really relevant to ask about what lens you may need if you aren't actually going to get one.
so...IF you have a lens/flash budget we will need to know what it is before we can recommend much.
your 18-135 will cover most of the focal range you will need for a small wedding, assuming you don't have to cover a huge area. its a slow lens however, and low lighting may present a problem for it. a good hot shoe flash/diffuser, if you don't already have one, will make a bigger difference in getting good shots that aren't far away than any new lens will. just make sure you get a softbox for it or learn how to bounce so you aren't flashing straight on.
if you are on a budget, consider getting a 50mm 1.8 for any low light shots you might need to take. its about as cheap as lenses get, and anything after that will just start getting more expensive.