Do you have a specific genre to work with? As far as I know, declamation selections can be poetry, prose, speech, etc. If fictional prose or poetry/drama is allowed, I'd suggest the following:
From To Kill a Mockingbird: Atticus Finch's closing statements in the Tom Robinson trial
From Macbeth: Macbeth's "Tomorrow, and tomorrow, and tomorrow" soliloquoy in Act 5, Scene 6
From Romeo and Juliet: The Friar's scolding of Romeo in 3.3
You can take this in many different ways, because there is so much from which to choose. I will talk about ancient works, because that is what I know best. Also, rhetoric and the art of declamation was in many ways developed in the ancient world.
If you go with the Greeks, you can do a part of a speech by Demosthenes, such as On the Crown, or his Philippics. This can be a lot of fun as you imagine the historical context of Athens and the rise of Philip of Macedon.
If you go with the Romans, you can do a speech by Cicero. He has written so much. So, there is a lot from which to choose.
If you go with a religious twist, you can read one of Paul's letters in the New Testament. Scholars are increasingly seeing the rhetorical elements in Paul.
If you go with a modern person, I suggest a speech by Churchill or Lincoln.
how do i love thee