I have this problem. Or, rather, I used to have this problem: I would go to the trouble to make a latte before I left the house, then I would get to Crossfit, and then drive to work, and by the time I got into the office and settled down to drink the coffee, it would be sorrowfully cold.
Clearly, I needed assistance.
Enter Coffeegeek.com, a heady resource for all things coffee related. Well, I entered "thermos reviews", and came across the Thermos Nissan backpack bottle. Reasonably priced, and it appeared to be well constructed.
I love it. It keeps my coffee pretty much at the temperature I finish making it (roughly 150 deg F), until I am ready for it.
It doesn't leak, at least not if you screw on the lid correctly. That is an important note - make sure you do it right! There is a little metal loop that lets you hitch it over the top part of the lid and it keeps the leakproof lid shut even if the spring-release button gets pushed inside your bag.
Conclusion? $30 well spent.