In the house I grew up in, we had a window in our shower. My Dad installed a standard Shower Curtain Bar from end-to-end to accommodate a curtain. And then my Mom hung a standard Shower Curtain on it.
You see, the problem with trying to install a curtain rod on the INSIDE of the window opening is that the 'setup' doesn't keep the water out. Sooooo, you have to extend the curtain beyond the boundary of the window opening.
If you go this route, you simply buy a regular shower curtain. If you don't want it to be as long, you'll have to cut it, I guess. If you need to professionally 'alter' the curtain, visit a local fabric store. Heck, maybe they can make one for you but you'll need the dimensions.