Did you at least get your money back from Selmer?
As for the rounded tuning slide, I think that you can get them from Blackburn and there is another guy online that sells them.
This is an online source for a "rounded" tuning slide, although it isn't going to be the single radius slide that I think you are looking for:
http://www.electrotheremin.com/myslide.html
Hope that helps.