I think one instance that clearly displays the theme of sacrifice is in Act III scene 3, where Banquo is shown to sacrifice himself so that his son may go free and carry on Banquo's line in spite of Macbeth's attempts to secure his throne against the prophecies of the witches. As the Murderers surround Banquo and Fleance, note how Banquo responds and what he says:
O, treachery! Fly, good Fleance, fly, fly, fly!
Thou may'st revenge--O slave!
The majority of productions show Banquo tackling the Murderers himself whilst he utters these final lines before he dies, so that Fleance has an escape route away to safety. This of course shows us a lot about Banquo's character in comparison with Macbeth's character. Throughout the play, Banquo is used as a foil to exaggerate Macbeth's evil, and here we see Banquo's goodness and self-sacrificial nature in his willingness to die himself so that his son may live.