Are Articuno and Zapdos obtainable ? I have only found Moltres so far. If they are, will they come in a random encounter like Moltres or are located somewhere?

Also, where do I find Regirock? Already got Regice and Registeel, but I just can't find him nor wake Regigigas without him...

And where do I find the ticket to go to Mew Island?

Articuno and Zapdos are roaming legendaries, like Moltres.

Regirock can be found in the Moonside Lake.

The Mew ticket is probably either the 100k TP, 200k TP or 50 Golden Dildo reward. I can't remember.