Where to get shieldon on platinum?

Just as in Pearl, you've to dig up an Armor Fossil from underground, but because this is the third game, it doesn't depend on which version you get (In Diamond, you rather dig up a Skull Fossil, of a Cranidos).

It's depends on your ID number now. When you get an odd number (ends with 1, 3, 5, 7 or 9), you can ONLY dig up Skull Fossils. When you have an even number (ends with 0, 2, 4, 6 or 8), you'll dig up Armor Fossils instead.

If you get a fossil, you know the story. Go to the Oreburgh Museum, talk to the righter guy in the lefter-upper corner at the bar, that accepts the fossil, then go in and out, and accept the Pokémon. At least, when you have enough space in your Party.