Disclaimer: I have only written half a plugin in my life.
What you can do is use the Custom Fields section in the Write Post page to add “metadata” to a post. For example, in the Key field I would write Source, and in the Value field I would post the source URL or bibliography.
You can then write a plugin that uses the get_post_meta() function to see if a particular post has Source metadata and use the the_content hook to display the source at the end of the post.
I’m sorry for sounding intimidating, but I don’t know of any plugins that do that. Actually, the half-plugin that I’ve written is called “Match Scores” which does a similar thing (plugin unpublished at the moment). It adds a form to the Write Post form and prompts for football team names and scores for a match. It then hooks into the_content and format the scores nicely.
If you like, I can send you a copy of what I’ve written in the plugin so far and maybe even customise it for your purposes. If you’re interested, drop by me at http://narnia.bounceme.net/jeremy/about/ and send me an email if you have a Wikipedia account, or an IM if you don’t.
