Take a normal shower curtain and have at it with a long kitchen knife. The knife cuts won't let very much water through so it will still be effective as a shower curtain.
Take a stray can of red paint and spray be a few two or three long strokes across it.
Only attach the rings of one end to the shower rail up to one half at most. Break the remaining rings and leave them on the bathroom floor.
I once saw a local Zillow listing with a very graphic gay bdsm shower curtain. So you can buy that somewhere, but that's a Google search I am not about to make for you.
Apparently they're closed now, but Artscow used to offer to print any image you provided on a shower curtain for you. I'm sure other alternatives exist these days.
A shower curtain of a very important, and well known document from your country such as the Charter of Rights and Freedoms from Canada, or Declaration of Independence from the US. If you live in a country that still recognizes the British monarch, you could also put up a photo of the king (very common in government and military buildings)