Chances are you need only a reducing adapter to change from the standard 1/2 inch valve to a 3/8 inch flex line. The adapter will probably be a compression fitting that will be easily available from a local hardware store. Just describe what you are trying to do. You may not even need a reducer if you have the right flex supply line equipped with a 1/2″ fitting to fit on the valve at one end, and 3/8″ thread where it attached to the dryer. Or you could use the standard 1/2 1/2 flexible connector and use the 1/2 3/8 appliance connector reducer at the appliance side.

There are some good graphics and advice at this site: