The Messiah is accessible on the web in score and parts for most numbers. 

For the aria 'Rejoice', however, I have only found parts for the 12/8 version, which was probably only used for the first performance in Dublin - whereas the commonly used 4/4-version is accessible in score, but not in parts. 

The score is (except for eventual misprints) a copy of the CCARH score, and the parts are extracted from the score. Feel free to use them!
Anthon Hansen (www.anthonhansen.dk)