Check out this other Visualize forum thread for helpful places to look:
You can always google image search what you are looking for as well. For example, google search for "interior room hdri free"
Then using Photoshop or other photo editing software, you can convert the JPGs into .HDR files for Visualize to consume. It's an easy workflow that can get you a ton of environment options in a pinch.
We are still working on finalizing the additional 20+ HDRs and matching backplates that we will upload to the Visualize Cloud Library.
More on that in the coming weeks.