We found the lanterns at our local Michael's and they were totally inexpensive. We purchased a couple lanterns, added some foam shapes and stickers to the mix and went home and decorated them.
Our son had fun bringing his fish-themed lantern to fruition. We let him go nuts with the stickers -- at only $2.00 a lantern, we figured he could decorate it any way he saw fit.
Once the lanterns were done, we hung them up in his play room where he proudly announces to everyone that he decorated the cool lanterns.