Attempting to bottle feed

Ok so I've been pumping and have a few 5oz bottles stored in the freezer. I would like to incorporate bottle feeding so we can have my parents babysit next weekend but I'm struggling.. she's ok with the bottle she'll take it but I can't figure out how to have a bottle warmed and ready when she's hungry. It's so much easier for me to just whip out the boob and give it to her when she's crying. Even if I keep a bottle in the fridge it takes forever to warm up running under warm water. I tried leaving only 3oz in the bottle so it's less full and I'm not throwing out the rest. I don't know I just feel like an idiot that I can't get this to work smoothly without feeling like I'm letting milk to bad or wasting it- also I'm poor and I really don't want to buy a bottle warmer unless it's really the only way to go