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It is available in two arrangements that I know of with Piano and soloist , also with brass band accomp ( I played it on tour in 1997 at the American Welsh Cymanfa Canu )
Thomas Wyss ( piano and solo) in the Thomas Wyss solo collection ( Kirklees music if I remember rightly)
Stephen Roberts ( while I remember I am sure there is a solo/ brass quintet version of Stephen's arrangement ) also with piano / brass band - Obrasso Publications
It is also recorded on the World Music Competition CD ( 2001) with Leslie Neish playing it with the Point Of Ayr Colliery Band

Largo al factotum can be found in most aria collections for baritone, as well as a single piece (e.g. here). Music for baritone and bass voices is usually written in bass clef, so you should be able to play the vocal part directly.

If I remember correctly, the range of the aria is second space C to high F.

I have an arrangement of Largo al factotum available on my website http://www.hammytuba.com in the music/solo Euphonium section. Hope this helps!

Look here for a large brass ensemble version of this piece: http://www.free-music-scores.com/large_brass.htm It is lovely to play and comes with a full score and all the parts.