I'd post a comment on anything you want to see the source code for. There isn't a way to search for it specificly afaik. If you wanna find projects with source code avaliable immediatly google open source gamemaker projects and you'll probably find a bunch of older, but largely complete games with the source code avaliable.

I found sandbox.yoyogames.com to be a good place to find old Game Source Codes. Just search for "engine" or "source code".