Hi nickall,welcome to the site.
This subject has come up before & I think you can only get rails that run the length of the roof for each side,not side to side.
You may be able to get some from another source but I`m not sure.
I've fitted the official roof bars to my 2006 C-Max. They go from side-to-side. You have to remove four plastic panels from the trim on the top of the car, then scrape off some sealant to get to the mounting screw holes underneath.
They're very simple to get on and off once you've put them together, but I did need a step-ladder due to the height of the car! Worth bearing in mind when you've got a roof-box on, as you'll need to bring a folding step or similar to stand on when getting stuff out at your destination.
When I removed the bars I put some stickers over the screw holes before putting the panels back in, to prevent water getting into the holes.