Welcome to the forum, Lou.
I'm not familiar with specific manufacturer that could make a pan to fit your needs, but if there is one, a pan of that size and shape would be quite expensive. For probably less than $100 in materials, you could make a shower pan out of deck mud. It would take you about a half day to get it all put in, and would likely save you several hundred dollars, if you think you could tackle it. Many others have done with no experience at all, and had good results with it.
Someone else might be familiar with a company that could make a pan like you're wanting.
Typically, a seat is built before the pan. You can do the pan first, but trying to build the bench on top of a sloped floor just makes for another challenge. My recommendation is to do the seat first.
Where would the pony wall go?
Your shower, due to its size and shape, is not going to lend itself to the installation very many prefabricated materials. Many custom showers rely on the installer to build the floor, benches, and sometimes even a soap dish or niche. It takes a little extra time, but you get exactly what you want instead of trying to work with something prefabricated that wasn't really made for that use.
