Tower Hobbies has, traditionally, been a Mini-Z dealer, though recent visits to their site shows a dwindling selection of items. They look to have plenty of iWaver stuff (I've been out of the hobby for a while, so I'm not terrible familiar with that product line).
One part, though small, that I got from Tower Hobbies that is interchangeable between the Mini-Z and the XMODS (and the Micro RS4) is aluminum wheel nuts.
If your looking for higher end, "custom" XMODS parts, Hobby Etc (
www.hobbyetc.com) is a repuatable, domestic hobby dealer that sells GPM items.