We just buy the 12 dollar "Thermos" brand at Target. All three of my kids say they keep their food really warm..sometimes "too hot".
We love thermoses for sending left overs to school. I have sent nacho cheese with a bag of chips, chicken nuggets, taco meat with a tortilla and sides in baggies also soup,chili or hot chocolate with a baggie of marshmallows. I personally love the versatility a thermos offers so the kids don't get stuck with the same ol' stuff.
Here is our trick..but really it is in the instructions. I just boil water in the morning then pour it in the thermoses and let it sit for about 10 minutes. Then I pour out the water and put in the warm soup or pasta etc.
I even send ice cream or yogurt in there also...but let ice cold water sit for about 10 minutes before filling it up.
Good luck and best wishes!