If your harmonica is out of tune, you could either:
1) Retune the flat reeds yourself - This is a difficult to do, and requires practice.
2) Buy a new harmonica - This is your easiest (although not your cheapest) option.
Harmonicas usually go out of tune in a few years of playing because of reed fatigue. How old are your harmonicas? If yours have gone flat in a few weeks or months, you might be playing them too hard. Playing softly will extend the life of the reeds.
Also see this article: