Drop by your local Wal-mart and purchase Bell brand Airtight 27-1 1/4 tubes. Don’t make a mistake and purchase the cheaper standard 27 X 1 1/4 bicycle tubes also on the shelf. They will fit perfectly in your 700 X 38 tires and have Shrader valves. These tubes are tough and hold air much better than standard bicycle tubes. The only downside is there weight, they weigh three times more than standard bicycle tubes simply because there three times thicker. I use them on my touring bike and have worn out three pairs of tires having traveled over 3,000 miles and have never had a flat. They only lose about 5 lbs of air in about 2 weeks where as standard bicycle tubes will lose that same 5 lbs in a couple of days.