did the stormtroopers rent out the dewbacks

Well, it's possible that the Empire has mounts available on Star Destroyers for different terrain, but I'd guess that, yeah, they acquired the dewbacks on Tatooine. Now, whether they rented them out or Darth Vader just demanded them is a different question.

Dewbacks were domesticated animals. Basically they were what horses are on Earth.

Granted the empire was vast, there were many supply chain mechanisms/systems at work to keep things afloat. So, I imagine so for a prenegotiated price.