Well it really depends what your kind of after.
What wood? what size? etc...
I know that there are slotted 6 and 8 strings. But there are no brands out there that build a 4 string slotted headstock and mass product it. So its going to be impossible to actually find one on the internet or elsewhere unless you get it Custom built.
What i know is that our friendly moderator "Nuke" has a funny obsession about slotted headstocks. He actually bought a simple uke that cost him 50 bucks,did a DIY attempt and looked superb!
He posted a thread about it, if you want to read it and see some pictures and even a video click
here
I've been looking for a tenor ukulele that suited my needs..i i wanted the different headstock idea aswell. I was lucky to meet someone that builds ukuleles. I had a chat with him about the design, what wood to use, fretboard, strings, headstock, size, electric etc...
He was more than happy to build it for me.
It will probably be finished in say a month or so, but he will be emailing me pictures for the progress and im more than happy to show u the end result when i receive it!
hope that helped.