Where can I find this lego brick?
I have one but I want some more of them for a project.
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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.
Could it be this one: http://www.bricklink.com/catalogItem.asp?P=3001f1 ? In translucent yellow it sells for about $2 a piece and is also quite rare.
Could this be what you're looking for?