Everyone says he was working for the Weller Bothers from 1838, but no-one produces documentary proof of that. I suspect it is one of those things he told everyone and it has simply been repeated unquestioned.
However, the Hocken Library has a number of documents relating to the Weller Brothers. One such document is the hand-written Journal of Octavius Harwood. It contains an entry for January 18, 1841, which refers to [Garret Hopper] Clearwater. This is the earliest reference yet cited for the presence of Garrett at the settlement before the immigrants arrived in 1848. It also supports the 1838 contention.