You get one from Prof. R's assistant just before you go into to Mt. Coronet, get all 8 badges+ deafeat the Elite 4 then go back to Jubilife and talk to the poketch guy contiuously, there's a guy in Sunnyshore that gives out poketch apps if you bring him the type of pokepersonality he asks for. There might be more places but as far as I'm concerned those are all of them.
Chat with our AI personalities
(I'm not sure if it's the same for diamond as it is for pearl) Occasionally visit the pokewatch maker in oreburg (?) city and he will have, hopefully, made new apps. And be sure to talk to everyone on your way, especially in houses, because they will also provide new apps. I don't know if there's an AR code for this....don't think so.
the most you can get is in jubilife, in the pokewatch building after you defeated the elite 4.
You can't get all the legendaries in Pokemon Platinum but I might trade you one. Contact me at likefruit89@yahoo.com.
kill all her pokemon.
i reckon PLatinum. it has all the new features but doesnt allow some Pokemon to appear in the wild that do in Diamond/ pearl. but platinum also has even rarer Pokemon that don't appear in pearl/diamond. I have all three. YAY