I’ve found that all my local flooring stores have extra vinyl flooring, left over from oddly shaped rooms or something. They are typically more than happy to get rid of this, and you can call in to ask if they have any available that catches your eye. Nearly always free and you can select a piece that blends in to your carpet or decor. I would also suggest getting a piece slightly larger than your x-pen to accommodate spills.
On top of the vinyl, you could place puzzle mats or yoga mats; basically some sort of cushion to protect your bun’s sensitive hocks! And then on top of all of that, you can spread out some attractive fleece or, my favorite, a stretch of canvas that is very discreet and looks good with basically everything. You can probably find this online, I forget where I got mine but it may have been Home Depot or Lowe’s or a similar hardware store, and I believe it was reasonably priced. You can always get extra so that when one is being washed, there is always something available.
I like to secure the fabric to the pen with binder clips.
Hope this helped!