According to the main entry point for getting started (http://confluence.jetbrains.com/display/IDEADEV/Getting+Started+with+Plugin+Development) the recommendation is to checkout the intellij-community source git repo to get the sources for the open api. However, this repo is enormous! I gave up on the git checkout and just went for a snapshot zip which itself was nearly 400MB. I'm sure a lot of this size is from the included dependency jars and tons of other things not needed for reading the openapi source.
Ideally, we could just have source jars for each released version which would be much quicker to download, and easier to find the version you need.
If they actually are available somewhere, I guess my google-fu isn't strong enough. The best I could find was the source jar from version 7 on maven here: http://mvnrepository.com/artifact/com.intellij/openapi