You don't really need an 18" barrel. 14" would do just fine.
You can find some at:
www.888paintball.com
www.redzcomfort.com
www.macdev.net
888paintball sells single 1 piece barrels, and the other two sell barrel kits that have spyder threading. you can also go to your local paintball pro shop and i'm sure they could help you as well.