Since you claim to already have Darkrai and Arceus, I would have thought Magmortar mere child's play. Color me surprised. Google is your friend. Magmarizer: Evolves Magmar into Magmortar when equipped and traded. Get from Wild Magby (Post-National Dex; LeafGreen must be in GBA slot to encounter; more common in Diamond).
Apparently you can also get one through the Mystery Gift process on Battle Revolution by entering the BA16-X4SH-E2AT code and sending it to Diamond/Pearl. seribii.net has more.
