Hi Kristinka,

Yes, you must purchase this music. It is not yet in the public domain and the composer and publisher should get paid when you use it.

You can order it here: https://www.jwpepper.com/3217924.item#.WcO0vob3arU

Or if you are in Europe you could try this: https://www.scorestore.co.uk/choral/dormi-jesu-by-rutter-published-by-oxford-university-press-oup.html

God bless.

thanks a lot!)