Hi, over the years I've had various cars and used Thule roof bars. The attachments have been the same and I've only had to buy the 'foot pack' which is car specific.
I was fortunate to have the bars long enough to fit the width of the my cars - you don't necessarily have to use the recommended length of bar.
You can buy all parts separately at 'The Roof Box Company', easily found on the 'web and helpful if contacted.