Newbie with a charging/battery question.

[dab-123]

I have developed a problem with my 2001 gz250. The last two times I have ridden the bike the battery runs down while I'm riding. I charge the battery with my battery tender until fully charged, but the next time I ride the battery will die and the bike will quit running when the battery completely dies. I suspect the stator, and was wondering where the best place to get a stator is, what they will cost, and how hard they are to replace? I'm fairly mechanically minded and good with my hands, I just don't have much experience working on bikes, as I have only been riding for about a year.Any advice is appreciated.