Waterfall actions aren't as common as they used to be... some people even mistake lipped keys for waterfall. M-Audio is one of the few companies that even does lipped keys nowadays, as diving boards are so much cheaper to make.
If your organist would be okay with non-sharp lipped keys, there are a few options that might work for you. Nearly all keyboards made by Nektar and StudioLogic have lipped keys. The Novation Impulse series does too, and the Alesis VI series and M-Audio CODE series.
Unfortunately, actual waterfall keys are a lot more rare. Some possibilities:This double manual monstrosity from GSiThe StudioLogic Numa Compact 2x
, which is not a controller primarily but might have waterfall keys (the Compact 2 is lightly lipped but the 2x is designed for organ use and might be waterfall).
I hope this helps.