I contacted auction-tokyo about getting a set in from Japan but it's working out very expensive.
From what I can gather there are two types that (probably) do the job, these & these.
I was quoted £300 to get a set of the first type delivered, that's £100 for the rails and £200 between fees & postage!
The proper rails would be a neater job but at that price I'm inclined towards getting some kind of bracket made up instead.
Here's a pic of the seats,
