Where to find this green transparent brick?
Your brick appears to be trans-neon-green, an extremely rare color for 2x4 bricks. It never appeared in any set and it is only traded between collectors. You would be paying over $50 for a brick like that if you are lucky enough to find it. It would probably be best to pick another color, or another brick-size for your project. Trans-neon-green also comes in 1x2 size, which is more readily available.