Looks like they're still in stock here:

http://www.starship-fantasy.com/ramps/Whitewater-Boulders

I got my last set from Marco Specialties, which I'm assuming got them from Starship, since they're the same shade of brown. There's a guy in Europe making these, I believe, but they're super dark brown, and not at all the same shade as the originals.

Edit: Sorry, looks like they're out of stock there too. Sorry. Still, Starship Fantasy is who usually makes these for distribution I believe. Perhaps contact them to see if there is a plan to remake them anytime soon.

Agree. I also have a set from starship. They’re excellent reproductions. Nearly identical to the originals in every way—except they’re definitely stronger than the originals, and perhaps very slightly lighter in shade (but I’m talking a negligible amount).

Definitely hold out for them to be remade.