Casteline or different types of sculpting wax are popular for bronze pieces which use loss wax molds where the wax sculpture is covered in plaster and then the piece is heated till the wax is all gone followed with pouring in the bronze and busting the piece out of the mold. The only thing with that is that you loose your original piece then, if I were doing a bronze piece I would mold the original sculpt wich could be any medium and cast it in wax then follow the process so I could make more than one piece -
www.sculpt.com is a good resource for wax materials if your looking to go that route -
www.smooth-on.com is a good resource for molding materials - wwww.dickblick.com has some good sculpting clays to work with if you decide to go that route with sculpting the piece out and then casting in wax before the bronze process. If you go with the scupting clay (the stuff that dries out, not the oil based) make sure to let the piece dry completely and then seal with spray on laquer before molding or you'll damage your original and have one messed up mold. This should get you started, any more questions I'd be happy to help out.