You should be able to get a 1 inch head set at just about any local bike shop, that sells BMX bikes and
Beach Cruisers. Here is a link to some information on headset sizing Sheldon Brown's Bicycle Glossary Ha-Hi
The thing to note is that there are several different 1 inch headsets. The one you need is probably the BMX/OPC assuming that the bike you are using has a one piece crank set? If you bike has a 3-piece crankset you would need the 1" ISO headset.