Probably not going to find a store carrying N900 parts unless you find a highly specialized cell-phone store.

Fortunately the N900 uses the same battery as the 5800XM (i think). If that is the case, you can try any T-Mobile store as they (used to) carry the 5800 so should have spare batteries for sale.

At least you would think so...

Are you sure the battery just does not need an overnight charge from an AC outlet? I heard that if the battery gets to the point of switching off it can be difficult to get it to charge.