Pssst, it's not mike, that's short for michael, it's mic i.e. microphone
Anyway, any old PC mic will do just make sure it's specifically a PC mic not a karaoke one or something. I got one for around $10 and it was perfect. Dunno the make it's a cheap one. Also I don't recommend sending .wav files. Get one of the 2 million free wav -> MP3 converters and convert your dictated files to MP3. This will reduce the size by about a factor of 10 if you choose the correct bit rates. You shouldn't need more than around 32kbps or less for voice. If your sister has windows media player then she can also play MP3s.