In what I think is a somewhat unusual move for scholarly archives (correct me if I'm mistaken), chunks of the Robert Heinlein archives are purchasable online. (You can apply for a research grant from them that might give you gratis access for specific purposes.)
The reason why they charge is given in a bracingly Heinleinian fashion in the FAQ:
Why isn't this free?
Short answer: TANSTAAFL
Long answer: Putting the Heinlein Archives online has been a huge and expensive task, from the cost of scanning hundreds of thousands of pages of documents, to creating the website, indexing the Archives, and creating the document delivery system. There are also ongoing expenses to host the site and maintain the dedicated server.
Though the Archives is provided online for research and academic purposes, The Heinlein Prize Trust, Robert and Virginia Heinlein's estate, who made the online Archives possible is not a non-profit organization. Just as Heinlein always said he wrote for money (something you'll find is true if you read through his correspondence), the Trustees have a responsibility to not only maintain, but increase the income of the Heinleins' estate.
Real good for free, see my August-September 2007 Reason magazine feature on Heinlein's career and influence, and my July American Conservative story on the relative influence of fiction on libertarians and conservatives, in which Heinlein plays a big role.