Can you trade a Mew in Pokemon Ruby that you got using an Action Replay in Pokemon Emerald?
You can get some Pokemon in Pokemon Diamond itself, but the rest, you will have to trade. The Global Trade System will help. Or if you can't help it, get an Action Replay. Action Replay is the easiest way. Action replay is worth it to buy. very useful. You can't you will have to trade from other versions :) or import from ruby/Sapphire/Emerald/Leaf green/Fire red