I take it these are to replace the 14" steel wheels+plastic wheelcovers?
14" alloys were available as dealer-fit options only but may (if I'm correct) fit the same tyres as the 14" steel rims (please correct me guys if this is wrong). If this is the case the ride may not be affected.
The 15" wheels as fitted to some models at factory would require a tyre swap.
Dealers will sell you alloys as accessories or 'replacement parts', even the older factory-fit 15" type fitted to SE Sport models. The other alternative is ebay.
If you are thinking of swapping 14" for 15", satisfiy yourself that the ride quality is still acceptable. Even on the latest cars, there is a noticable increase in road noise and slightly more firmness to the ride compared with 14" wheels.