Since the mode can of worms has been opened, here's how to find which harp you need to play key X of mode Y:

Major/Ionian: you're there; to play B major, you need a B harp.
Dorian: move <= 2 half-steps; scale starts one diatonic note higher.
   To play A Dorian, you need a G harp (2 half-steps down from A)
Phrygian: move <= 4 half-steps; starts two notes higher
Lydian: move <= 5 half-steps; starts three notes higher
Mixolydian: move => 5 half-steps; starts three notes lower
Minor/Aeolian: move => 3 half-steps; starts two notes lower
Locrian: move => 1 half-step; starts one note lower (like you'll ever need this)

So, to play G Dorian, you move 2 left from G; you need an F harp, and (on a standard or Paddy) the scale would start on draw 4 rather than blow 4. To play E Mixolydian, move 5 right from E; you need an A harp, and the scale starts on draw 3 rather than blow 4. (On a Melody Maker, the major scale starts on draw 2 or blow 6, so you figure the new scale start relative to these.)