Try AliExpress or Ebay, there are some different models around. But most are regular TPU/silicone cases, not that different from the brand case. Some of these just have their backs printed. Some are shock protective with reinforced corners, some wallet cases, etc.
I'm used to flipcases and there are some available now. But I liked none. They all felt cheap. I'm using the official FP4 flipcase now on the FP5, it fits almost perfectly and is the best option I found, so far.