ear training resources?

I've been tasked in training my church vocalists on their hearing.

They don't have the best pitching around and I'm not experienced in training per se. I'd like to invest in good resources to help singers sharpen their ears, leading to learning in hearing harmony parts from music tracks or even from music chords, e.g. when the band changes some chords in the arrangement, they'd be able to hear and pick out the harmony notes.

Any suggestions would be great!

Thanks!