Pick some of the densest, hardest woods in your area. Of course, you can always use lumber. If you want to make a flatbow, just about any wood as denser than cherry will work (so will juniper, sassafras, and cherry too). For an actual Elnglish longbow most of these woods will work too, jut leave it slightly wider for second string woods (1 3/8" wide) and longer (as tall as you are). There are lots of examples of various wooden bows here, especially the Bow of the Month.