I got a "hot water carafe" on the change table at home (extravagant I know) but DH fills it with warm water in the morning and DD get a warm bum wipe all day!! Pop the washer in the plastic bowl and pour a bit of water in and your set to go.
For out and about just keep a small water bottle in the change bag - those ones they give you on airplanes are a great size or the kids poptops juice things are great and you can pop it open, squirt some water out and off you go - but they can leak occasionally if you don't keep them upright so a screw on lid is better.