I built one. I got my parts from www.pixcontroller.com, but they stopped selling the components last april. There should be other companies selling parts for home brew trail cameras. In a goodle seach I found this one near the top.
http://www.fieldpix.com/
I am sure there is a forum or two out there just for those building these where you can get ideas and help.

To me, game cameras have gotten cheap enough and advanced enough that it is not worth it money wise to build my own anymore. But it can be rewarding knowing you designed and built one that does exactly what "you" want it to do.