It might help if you tell us the OS, development tools, and graphics library (OpenGL, DirectX, ...?).
"game source code c++" does give 4+ million hits in Google but including the OS + development tools will narrow it down.
Windows: CodeProject.com has some, MSDN.Microsoft.com has some for the XNA cross platform library (PC + 360 + Zune (next version of XNA)).
There are a bunch of game programming books with source code, often with a simple 2D and/or 3D engine included.
www.Gamasutra.com -- poke around for code links