While I am unaware of any canonized saints who the Church would not consider in union with the Catholic faith, I do know that we share a number of saints with the Eastern and Oriental Orthodox Churches. These primarily include the early Church Fathers such as Ignatius of Antioch, Augustine of Hippo, John Chrysostom, Irenaeus of Lyons, and many others from the first millennium of Christianity.
Of course, this deals with those who are recognized publicly by the Church as saints worthy of veneration – those who are in “the Christian Hall of Fame,” if you will. We admit of countless others who are “uncanonized” saints, those faithful Christians who have entered into the presence of God for all eternity. Many see these referred to in Revelation 7:9, as the “great multitude, which no one could count, from every nation, race, people, and tongue. They stood before the throne and before the Lamb, wearing white robes and holding palm branches in their hands.”
But is it possible for someone who is not Catholic or even Christian to be saved in an extraordinary way? Here it is important to distinguish between the ordinary and extraordinary means of salvation. The ordinary means of salvation is obviously through Christ and his Church. But the fathers of the Second Vatican Council in Lumen Gentium, the Dogmatic Constitution on the Church, (paragraph 16) admitted the possibility of extraordinary grace when they noted that “Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience – those too may achieve eternal salvation.” This principle can also be found in the Catechism of the Catholic Church, no. 847.
So while there are no non-Catholics that the Church recognizes as saints, the Church does recognize the possibility that non-Catholics and even non-Christians can be saved through the universal power of Christ’s passion, death and resurrection.