Amazon.com: Lego Parts: Electric, Light Brick 2 x 3 x 1 1/3 with Trans-Clear Top and Yellow LED Light
BrickLink - Part 54930c02 : LEGO Electric, Light Brick 2 x 3 x 1 1/3 with Trans-Clear Top and Yellow LED Light (Glows Orange) [Electric, Light & Sound] - BrickLink Reference Catalog
LEGO Light Brick - LEGO Town - Eurobricks Forums
LEGO light bricks – more options!
BrickLink - Set 970-1 : LEGO Lighting Bricks [Universal Building Set:Supplemental] - BrickLink Reference Catalog