I didn't sterilize bottles when I was at WDW. For my first baby I sterilized bottles for a month or so but the pediatrician said I didn't have to unless I had well water. WDW doesn't have well water, so I wouldn't be concerned about any type of issues that would require you to sterilize bottles.
I also brought a small bottle of dish soap and washed the bottles in hot water in the bathroom sink. We did just fine. If you're not comfortable with not sterilizing your bottles, you can stop at a grocery store and buy premixed individual use bottles, but be warned that they are expensive!