FYI I looked up the local dealer for me. It looks like they do offer to sell you just the parts. Its not cheap though.
Brighter Concepts — Solatube Price List
I also emailed my step father (general handyman and DIYer) to see if he has heard of them. This is what he said:
"I have seen them. Gary and Brenda have one in their upstairs Bathroom. They do add light but nowhere near what a window does. A great option for a room without a window such as a bathroom. From what I've seen, are a bit of work to install. Especially since you need to cut open the roof and then are faced with flashing and sealing that opening."