Inexpensive saddle bags will work fine. As Hair says, just freeze your water bottles the night before 3/4 full and add water in the morning. They'll stay fine that way.
I've also found that wrapping them in a little newspaper will make them stay cool for much longer too. I just fold the paper so that it's only as wide as the bottle from the bottle up to the "shoulders" of the bottle, then roll the bottle up in it and put a rubber band on it. You'll still have ice in it even at the end of a long 90+ degree day. Not much of a consideration this time of year, but a nice trick for the summertime.