Crimson Trace released laser sight for Mossberg 500, 590, and 590 Shockwave shotguns.
LS-250 uses red laser,
LS-250G uses green laser, price at $169 and $219 respectively.
The sight can be used by either hand.
This is for 12 ga. model, not for 20 ga. or .410
From: Crimson Trace