C.B. Gitty is a great source for instrument parts. They just got some 4 string bridges in stock that would work for an electric ukulele. Between them and a wood store (like Rockler), you are only a couple of orders away from having all of the parts you need to build.
Here are some of the part that you need that are availible from C.B. Gitty. Keep in mind that some of the parts have multiple styles and colors. You can look around and find some that are perfect for your project.