Yeaah, they're still making the Superjet standup, but it's supposed to be sold for closed course use only. Kawasaki stopped making them in 2011.
HSR Benelli out of Austria still makes a standup ski seen below.
Also, OP, I forgot to mention Polaris made a standup called the Octane in the early 2000's. That's another tough one to find, but they generally go for cheap in comparison to a Superjet or SXR and they are piped stock and run 55 mph out of the box.
There's also the Sea Doo 3D, although people will look at you the same way they'd look at you if you were wearing snowblades on the mountain when you ride it.
Another option is a Kawasaki X2, which is kinda a combination between a standup and sitdown. They are 650 based, so really easily accessible parts, easy to work on, cheap, etc. They usually go around $500-$1000.