I need to do some costomization of the plugin, and if the answer is YES, where can I download the source file?
PS: I have been thinking that all of Atlassian products are open source
It's not open source per se (i.e. you don't have the rights to redistribute it) but, in conjunction with our standard practice, customers will be granted access the source code once they purchase the product. We'll make this available once we ship 1.0 after the new year. Please watch CQ-436 for updates.
No. Most of Atlassian's core products are NOT open source, and that includes Confluence Questions.
You are entitled to a copy of the source code if you have a commercial licence - that enables you to do customisations, but you absolutely can not share the code you download (or release products etc). You can log in to my.atlassian to download that source
I don't actually know about the Open Source licence. I don't think it does offer you the source, but I'm not sure.
You should be able to get the source from your my.atlassian account, so if you can't see the "download source" link under the licence, I'd say not.