Big Ben Interactive, who handled the N64 in Europe were selling them off with 2 games for £20 - not sure if they have any left but give them a ring and find out: 02380 653 377.
They also sell off old dreamcast games at a fiver a time!
You could also pre-order the new Zelda game - Wind Walker - which you get a free disc containing Ocarina of Time which is playable on the cube, with a much better frame rate.