I am using HtmlUnit to read content from a web site.
Everything works perfectly to the point where I am reading the content with:
HtmlDivision div = page.getHtmlElementById("my-id");
If by original HTML you mean the HTML code that HTMLUnit has already formatted then you can use
div.asXml(). Now, if you really are looking for the original HTML the server sent you then you won't find a way to do so (at least up to v2.14).
As a side note, you should probably think twice why you need the HTML code. HTMLUnit will let you get the data from the code, so there shouldn't be any need to store the source code but rather the information it is contained in it. Just my 2 cents.