Easy if you go to a Dealer, but I think its been costed before at circa £500
Alternatively, there are 2 or 3 Cromas being broken on ebay, but not sure if they originally had or still have the bars. This the route I went and got mine for £50.
You need the correct retaining bolts, which depend as to whether you have the glass roof or not, as if you have then they need to be a couple of mils shorter. They are not that expensive from the dealer, so no problem as to what spec car you get any second hand bars from. If you can, also get the black plastic strips that run the length of each side of the roof - as eper shows they are different if you have the rails. However, I don't think these are essential as there are inserts in the standard strips that pull out where the bars fit