That’s a good question, WJ; I haven’t seen them around much lately. Some German restaurants, such as Mader’s and Kegel’s Inn, may have it, but their online menus don’t list desserts. You may want to give them a call to find out.
The Range Line Inn in Mequon (on Mequon Rd.) is a restaurant that has Strawberry Schaum Torte. You could get it to go, I’m sure.
For bakeries, you may want to call C. Adams and Foltz Family Market–both in the downtown Milwaukee Public Market–to see if they have it. I know that the predecessor to Foltz, Nehring Family Market, had Schaum Torte on offer (as well as Bienenstichkuchen), but not sure if the new owners still do that. The Nehrings still own Groppi’s and the Sendiks on Downer, but I haven’t seen Schaum Torte at either place. Still, it may be worth calling.
There are some other bakeries that may have it–Simma’s in Tosa, perhaps. I doubt Grebe’s and National Bakery would have it, but you can always call to verify.
You may also want to check the larger Sendiks supermarkets (not owned by the Nehrings), such as the one on Layton Ave. (near Kopps) in Greenfield, or the one on Port Washington Rd. in Mequon. They have an extended bakery selection of cake slices and such, and I wouldn’t be surprised if they have Schaum Torte.
If Sendiks doesn’t have it, I doubt Metro Market or Cermak would have it, but it may be worth a try to contact them.
If I think of other ides, I’ll let you know. Maybe MFL or another poster has some insights to follow.