uh... anywhere! Bora Bora in Playa d'en Bossa, the bars in the Ibiza Town harbor area, the sunset bars in San Antonio... these are the places you generally find people pre-party (and where you can pick up your club tickets).
And, of course, you'll find 'like-minded' people at the clubs. If you pick your parties based on your music tastes, you'll even have a good chance of finding people that like the same variety of house that you do. Then again, I've rarely met anyone at the clubs (not in proper shape for conversation) so you're better off trying to make friends at the pre-party stage.
And there's always the beach and your hotel bar...