Aaah, okay, I definitely misinterpreted your question - I was thinking Pokemon art for Gym-era cards! Finding similar sort of pre-made interiors for Gym/Stadiums would require a bit of searching on DeviantArt and/or Google, perhaps with search terms such as "interior", "3D", "stock", "free", "background/backdrop" and the like. One of our past fakers, @kachitta, who you might remember, has made some neat gym interior art using Photoshop, with some examples in their gallery. I asked her how she did it once, and these were the tips she gave:Really any style would work, but what I'm looking for I've been having difficulty finding. Basically I'm looking for interiors of gyms for Stadium cards from the Johto region. So like for a Blackthorn City Gym card and such.
Some tips? I have prepared bitmaps with gym field and gym sign in their normal shape. I copy them to some real-life image and transform them using Scale, Perspective and Cage Transform tools to fit onto the surface in the image. Then I copy the base image into the Layer Mask of the inserted layer to apply the texture, sometimes it's needed to adjust Brightness-Contrast of the mask. When the surface is jagged, I split the inserted layer in several ones and transform them using Cage Transform until I have a feeling of reality. Is it clear?
I haven't tried it myself, but it sounds somewhat basic yet effective. I think there's a lot of room for this technique to be experimented with so that the stock can be manipulated to feel more "gym-like", beyond just pasting a gym field and sign on real-life places. The difficulty here would be finding real-life, free-to-use interior images that work well. Some of my favourite free stock image sites are Unsplash and Pixabay, but I'm not sure if they would have anything you'd be looking for.
The other option, if you're looking for creating your own art, would be to use the 3D design engine Unity. @BigfootAUS was experimenting with this a lot when he was around, but learning to use the engine, and then using it well, can both be significant learning curves.