the charger you need might be one that you already have. To bring the batts up to best possible condition they should be charged at c/10 for 15 hrs.

An old wall charger that doesn't shut off is best for this. You can use 200-800 ma and figure the time out from there. Ideally around 300 ma and 500 ma is what to look for. They are vey cheap. lots of people have them laying around not used.