There is someone on the sprinter-source forum that installed sliding T5 seats in his sprinter.
He ordered them from overseas.
Sorry I don't have a link.
The rails are built into the floor of the van. Pretty sure you'd have to get them separately from the bed though VW does use them in most variants of the vans for regular seats, tables etc as well. Try junk yards in Europe maybe. That and using a freight consolidator so you get close to container shipping prices might make it worth while. Or, maybe they have them in Canada?