for now only gen 1 trough 4 Pokémon are available, the pseudo legendaries are rarer overall than other Pokémon but can be found in the wild. For this set in particular, i would recommend playing in a regular forest or jungle, where Pokémon spawn rate is high, and hope you are lucky.
If you want to quickly renew the wild Pokémon on your map, going on hunting parties to clear the uneeded wild Pokémon already there will make new ones appear faster
Also, going on quests that lets you play on maps in other part of the world is a good way to find new Pokémon.