I breastfeed too, but also pump bottles for my mom and fiance, and when we are out. I won't BF in public ;)
We went out shopping today. Put some milk in a cooler bag and when we were ready to feed her, we got a cup of hot water from the grocery store and used that. I fed her, her bottle in the backseat of the car. It worked!
Husbands armpit! ? haha! For the most part I bring ready to feed formula and sterilized bottles when we are on the go. If I have milk in the cooler bag I try to time it so it warms up just by sitting at room temp for a little while. They do make travel warmers but they take a long time..
Breastmilk actually doesn't need to be warmed, so if bub will drink it cold then no worries.