After you defeat the Elite Four you go back and talk to the professor and you will get one.
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After you get the Nationl Dex you can catch him in the Safari Zone after placing 50 objects.
Yes, if you evolve a Pokemon from the national dex while you are still using the sinnoh dex then all forms you have accessed previously, whatecver dex you are on, can be found on the dex. Hope this helps!
There are 210 in the Sinnoh Dex, 493 in the National Dex {including EVENT Pokemon}
You get the national dex when you beat Cynthia. And get the poke dex at Rowan's lab.
you compete the sinnoh dex.