Pouch cell is any cell with soft enclosure. They can be flat or they can be thick.
Would not you have to crack the case open top get it out? I depends on how it’s built. In Rovyvon f.e. you can open the body without damaging it.
Where to get replacement? As usually, in China. They are less standardized then cylindrical cells, so getting the perfect size is harder though. F.e. 501030 cell as used in the 1st gen rovyvons can be found here.
Naturally you may also ask the manufacturer but they are unlikely to keep stock of fresh cells after several years (when your original cell wears down).
But I find it a bit weird that on the other colors the side LEDs just shine through the colored body. And in the video it looks like it doesn’t work very well. If that wasn’t the case I would have taken a colored one, I think they look pretty cool.