there are only a few available for the micra , if its a thule , it will be written on , the one made by nissan isnt lockable , u could try doing an e bay search for it, to identify which one u have and the retail value.
i think replacement keys might be hard to get , u could possibly buy new locks with new keys as some of the locking mechanisms are easily swapped as long as they arent locked , and yours isnt .