This post is totally weird, but in the off-chance you are not about to try to sell us sumshit, I'll answer.
DH is on formula now, and depending on where we are, I will usually stop and get some hot water from a coffee place, and mix it with some cooler water (hot water at a coffee place is a billion degrees). I then mix in the formula and walla... bottle ready to go.
If we are just going out on errands and he'll need a bottle while we're out, I'll heat the water beforehand, put it in one of those thermal bottle pods, and then mix the bottle whenever he needs it.
Breastmilk (assuming it's not coming straight from the boob and has been pumped into a bottle) is totally different, because you need to keep it cold until it's time to heat it. When DS was still breastfed, we used a bottle warmer at home. While we were out, I'd put the bottle in a cup of hot water and let it rest for a while until it warmed up.
Travel bottle warmers suck and are an enormous waste of money.