There's one simple answer: trade! Cranidos is to Diamond as Shieldon is to Pearl, so find a friend who has Pearl and trade with them.
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The first trainer on the way to the Pokemon league has a luminion.
Beat the Pokemon League, and you will be able to find all but one of the fossils of the ancient Pokemon. (For Diamond, all but Shieldon's fossil are available)
There's no Trainer Cafe on Pokemon diamond. There is only one on Pokemon Platinum.
A Pokemon guide that you can find in the forest.
you need a cheat that can catch a trainer's Pokemon to have lapras. because there is a trainer that has a lapras.