How were you supposed to find the tournament codes in the original Mario Tennis?

I remember when I played Mario Tennis 64 that there was some tournaments that were blocked with codes like these.

https://www.ign.com/wikis/n64-cheats/Mario_Tennis_Cheats#Nintendo_Power_Cup

Now with the internet they are easy to get, but back then, how the hell were you supposed to find those codes? Were they given at events (Like Mew in early pokemons) or did you have any in-game way to find them out?

For events, I never heard of such events happening (even the Mew one I mentioned), but being from Canada (Quebec) I know those were often US based and not reach up here in the north.