I can give you a few sources to get some royalty-free liminal images. Quick question, are you using them in your game or just for inspiration? 

Here's some links I found:
1. https://unsplash.com/s/photos/liminal-space (Lots of high-quality pictures).
2. https://www.pinterest.com/search/pins/?q=Liminal%20Space&rs=typed (Some on here are royalty-free)
3. https://pixabay.com/images/search/liminal/ (Not much but still enough to get inspiration).

Extra: https://gamejolt.com/c/LiminalSpaces-pscazp (Great for inspiration)

Update: Do you have any experience making video games? Because my team could use some more people. You can find a role that fits your skills. I made you two lists you can pick from.

List 1 (Little to no skills needed, great for if your just getting started. There still very important)

I personally recommend Wikimedia Commons.

you can also create liminal images in blender! it’s very easy to do and if you render them at a lower quality and as jpegs it’ll look realistic :)