some by catching and by trade
No, you will need to trade for them or transfer them via Pokemon Bank to get them in Pokemon Omega Ruby and Alpha Sapphire.
Can't catch another. Trade it for 3 starter.
Catch It In Cinnabar Island
no omega is not a real pokemon, but arceus is considerd the omega pokemon.
You can find whismur at route 503, and mankey is currently a swarm pokemon, so you'll have to wait or trade for it.
Riolu is at the Haunted Zone (I think) I am not sure but I think he is at the haunted zone.
you can only find it during swarms
The Prism Scale cannot be obtained in Pokemon Omega Ruby and Alpha Sapphire. You'll need to trade for a Pokemon holding the Prism Scale to get it.
You will need to trade for Weedle or breed Kakuna. Kakuna can be found only in the Safari Zone in Pokemon Alpha Sapphire and Omega Ruby.
you can't find omega
Illumise in Pokemon Omega Ruby can be caught in Zorua at the park.