Two ways
Easy way: Migrate from FireRed/LeafGreen (Recieve at Lv 25 in Silph Co.)
No-Trading Way: Can be found in the Post-Dex area in Victory Road in the Water.
Chat with our AI personalities
you need a cheat that can catch a trainer's Pokemon to have lapras. because there is a trainer that has a lapras.
you find lapras in union cave B2 only on fridays
I don't think you can but if you have an Action Replay you could catch the trainers lapras.
146 = Moltres 131 = Lapras
neither of them is better, they are both the same, but in diamond, you catch dialga, and in pearl, you catch palkia, and you can catch different Pokemon in each game for example: you can catch this Pokemon in diamond, but not pearl but: you can catch this different Pokemon in pearl, but not diamond