To keep coffee hot you should buy a good insulated coffee mug which should easily keep the coffee hot for 4 hours or longer if you're drinking it throughout the day.
Or you can buy a good coffee thermos that will keep the coffee hot for 8 hours or longer if you need to carry the hot coffee around.
If you have a ceramic coffee mug on your desk you can use one of those electric candle warmers that plug into a 120 volt outlet.
I have one of those candle warms on my desk that I place my ceramic coffee mug on and it keeps my coffee or my cup of hot enough to drink.