I seriously have to try one of these myself, and the article mentions a few places in the country that make it, notably Lutz Pastry Shop in Chicago, as well as the Cake Box in Huntington Beach, California. Up in the Bay Area, I've found that they carry a Japanese version at the Marina Foods in Union City, and a German one at Lehr's German Specialities in San Francisco. It's no surprise Chowhounds have a thread that points to a few places you can buy it online as well. And there are even a few recipes scattered around the internet, though I don't know how you could replicate it without one of the special ovens; the Cake Box claims there's only seven of them in the U.S.
I did a YouTube search to see these spit ovens in action, and got a little more than I expected. The first two videos are pretty good at showing how the cake is made (note the awesome Polish dance music in the second one). The last one is just bizarre, I mean seriously, that's what you chose as your pet? And just to add some more media to this post, below is a Flickr slideshow of baumkuchen.