where can I find the transposer midi plug-in?

Look in the channel strip>MIDI FX. Click on the box and select from the menu. You can also transpose channels trips in the channel strip inspector. Select a channel strip, then in the channel strip inspector, select MIDI input.

Also don't forget about the existence of dedicated 3rd party MIDI transposing (Autotune for MIDI) software like AutoTonic Note Transposer which also works with Mainstage or any other mac or win based DAW, Virtual Instrument, Outboard Gear, MIDI destination etc. and it might provide even more sophisticated controls and also the option to manage any scales records in a Music Theory Library etc. etc. This is from an indie-developer from Vienna, and they hey have definitely put a lot of efforts into creating this … maybe it's something you should consider.