Although Carbon source code is widely available, a lot of code written prior to Mac OS X has been obsoleted by the introduction of Carbon (Apple's developer site contains a mixture of both pre-Carbon and Carbon source code, and so you should always be alert as to the age and version of the code you are looking at).
I like to have a folder with lots of source code in it for Spotlight searches. Type in any keyword or API function and find an example of that function is use. Spotlight is very helpful for programmers in indexing lot's of source code for future reference. see Spotlight Headers on how to index the developer examples.
Please, add to this list if you know of other repositories of Carbon source.
Most of the CarbonBooks provide sample source code. Much of it is free for downloading at the book's web site. So, even if you don't buy the books, you can look at the sample code.
There is a growing number of code snippets available on this web site. see Core Foundation and Control Manager. If you don't see what you want, please, create an entry and ask the question.
Recently Google has added a search engine for source code which helps you find function definitions and sample code by giving you one place to search publicly accessible source code hosted on the Internet.
Simple searching for Carbon API functions virtually always yields a couple of usable results.