To unlock Green Arrow, in level 4 (Space Station Infestation) during the part where Grundy is hiding in the armory there is a power station in the back part of the map that needs to be energized. Either use Shazam or find the full power station on the back upper level to charge the station and you will get a scepter. Take the scepter up to the top left to where Bat-Mite is and he will give you the Green Arrow character token.
Green Lantern you get during normal game play.
Green Arrow costs 300,000 studs. Purchase him in the character selector, select him and then select Green Lantern as your secondary character.
Edit: Thanks to StrongProtector for pointing out my oops on the level #.