When Harry and Draco met in PS/SS, Draco a dit Narcissa was buying him a wand and Lucius was getting his books. If the wand chooses the wizard, how could Narcissa get him a proper wand?

It is possible that Draco already looked at the wands and tried them out. Cissy might have just been buying him the wand that worked for him.

maybe she was getting him the exact same wand?? I don't remember that part-sorry :(

It is possible, since Draco's dad is a wizard and his mom is a witch, that he has had a wand before Hogwarts. Therefore, all Narcissa had to do was ask the wand-maker (sorry, don't remember his name)for the same kind of wand. Just like when Ron's wand broke and he got a new one without having to hold every one all over again and see which one worked for him.