What is your budget for onsen?
and are you looking specifically for room with private ensuite onsen bath, or are you okay with doing communal onsen baths ( which is what most people do )?
there are plenty of places where you can stay for about ¥50,000 per night for two people with private ensuite onsen tub, dinner and breakfast included. However, rates like that I will only be available on the weeknights ( i.e., not Friday or Saturday night). Also, at most places, rates get substantially higher from around December 28 through the first three days or so of January. December 26 or so maybe just about the best date to get the most reasonable rate for Onsen ryokan.
there are countless nice onsen places. So randomly emailing places here and there is probably not the most efficient way to find a place with a reasonable rates.