How long are you going for? I have used disposable bottles by a company called Vital Baby. I got them in Boots, no idea if you can get them elsewhere! Each bottle is individually wrapped, so they stay sterile till you need them.
It's basically like a bag that is attached to a teat. The liquid gives it rigidity, I never had any problems feeding my LO from these.
I tend to always carry one in my bag now - just in case