You get the Stone Prisoner free when you buy a new disc based copy.
Unfortunately you do not get it when buying the version from Games on Demand, so you would have to pay another 1200msp if you want it.
If you know anyone who owns the Ultimate Edition of the game though, you will be able to install all the DLC from the bonus disc. Renting the Ultimate Edition, installing the DLC and then returning would also be an option.
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