How to grant haste en masse in a mono-green Commander deck?
A simple search for cards with the words "creatures" and "haste" in their text boxes reveals a couple cards with Green color identity that grant mass haste:


Akroma's Memorial is an artifact that grants your creatures Haste, along with a few other abilities.
Concordant Crossroads grants all creatures Haste, not just yours.
Reflex Sliver gives all your Slivers Haste and Hivestone makes all your creatures Slivers, so together they give all your creatures Haste.