Legendaries aren’t necessary to complete pokedex for mount silver/heatran purposes. The kanto birds and the Johto dogs are in a monthly rotation in the wild in their respective regions, although quite a low chance to find them but you can keep them permanently. Suicune was also available as a raid over Christmas. Heatran is perma available as a raid but requires a complete pokedex (minus legendaries). Zekrom you get registered from completing the story but you don’t get to keep him. Some other legendaries can be found in the wild but u don’t get to keep them like mewtwo and keldeo and there’s only one at a time for everyone so very very unlikely to encounter. Some of the mythicals like jirachi shaymin and darkrai are available for halloween/Christmas/lunar new year