I am looking for an original topper also and willing to pay $300-$350 for a working waterfall decal. In the meantime I may just buy a reproduction without the waterfall effect. Way cheaper for now until someone can reproduce a working one and sell it for a normal non ridiculous price. There is some new hope. There's a hint at the bottom of the following thread and a YouTube video showing the material he may use to create the waterfall effect but who knows how long it may take.
https://pinside.com/pinball/forum/topic/3d-translites-next-titles-wh2o-and-afm