You are asking for two different things.
18's are really only good for show UNLESS you want to stuff a big brake kit inside the hoops.
In my dictionary ( go over show ) any aftermarket rim should weight the same or less than the OEM models. I won't go into suspension changes, power loss, braking impacts, etc.
So if you want that 16lb 18" rim they are out there. Just be ready to pay some good coin to get them. Least expensive I have seen are the Kosei TS models from Tire Rack here in the States.