This is a popular contemporary choral arrangement. You can download the PDF score for free via the Choral Public Domain Library (CPDL) 2. Antonio Caldara Version
By following these tips and being persistent in your search, you may be able to find the perfect Sanctus Missa Laetare partition PDF for your needs.
: Aim for a "Maestoso" (Majestic) feel that balances the holiness ( Sanctus ) with the Sunday's underlying joy.