There are several good solutions. I believe all of these have been mentioned already but I think I would like to add that I feel these are the most reasonable options.
The most popular one, in my experience of talking with other people who have the same problem as you, is to keep hot water in a vacuum thermos, then use that hot water throughout the day to brew tea in a teapot, cup, or mug. Here
is one example. Note in the description that it is a "double-wall vacuum insulation" type thermos. This is what you want - a double-wall thermos with a vacuum in between is just about the best insulation you can buy. I even have a friend who owns a model that has a small spout at the top. His model is designed to be used in exactly the way I've described here. This method is popular because you can take a thermos of hot water and a mug with you wherever you go, even without electricity.
The second option is to buy a hot plate
. These are basically individual electric stove top burners that you can plug in to an outlet. Your office may have rules regarding such devices, as they can be dangerous if left on accidentally. You can turn it on low and keep your teapot on top. As long as the heat is low enough, those consumer-grade iron "teapots" will be safe and keep the tea warm. Don't let the teapot sit on the hot plate while it is empty, as the pot can get too hot and that can damage it.
Of course your last (and in my opinion, best) option is just to buy an electric kettle and keep it at your desk. If your tea gets cold before drinking it, then likely you're brewing too much at once, and instead you should brew more frequently and less at a time. I think this is a refrain you will never stop hearing from teachat
Of course if you're moving around a lot during the day, it's cumbersome to take the kettle with you all the time.