Online sales through the Spotlight website will get you tickets to Space and Manumission.
For the other clubs/nights, you can usually find cheap tickets at the bars in Ibiza Town harbor, San Antonio and Playa d'en Bossa. Ibiza Town is my preferred option because sometimes you get lucky and find tix for less than the official price (though there might be conditions, like you have to arrive by 2am).
It's not necessary to buy your tickets way ahead of time because I haven't seen a party yet that "sold out".