You could try looking for Bunk Bed Bolts, but I bet you won't find that size or imperial thread. That type head probably won't hold the tensile strength of a 3/8" bolt which is why I think you won't find them. If you want hardware for furniture construction, the typical M6 or 1/4" diameter will be plenty strong. McFeely's has a good selection of 1/4-20 here. Don't be afraid of metric hardware.