That's one of the most popularly used Gregorian Alleluia's in the world. I don't have any accompaniment books in front of me, but it shouldn't be too hard to find. It's also very easy to write your own.
For example, using the bar divisions in the antiphon, create an accompaniment based on the antiphon's melody and the following chord progression to harmonize it. G____/D___CD/C__D_G//
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