There was only 1 demo disc for the public. And that one was intended to be only for people who bought a gamecube but some retailers sold them to everybody anyway. And they charged whatever they wanted to. IIRC Gamestop was charging $5. Some Charged $10. I think best buy initially charged $30 for it. Other than that it was just demo disc for store kiosks.
Nintendo (as far as I can remember) never gave any out thru Nintendo Power (like sony did w/ Offical Playstation Magazine). When I used to work in teh Electronics dept at Wal-Mart I brought up the idea once to the Nintendo Representive and she said they considered that but deemed it too costly. :/