If it's in a wheel, the easiest and usually cheapest option is to buy a new hub and swap the guts; axle, cones, bearings and freehub body. If it's not in a wheel, it's hard to get the freehub body off. Take it apart and see what's wrong. I've got an axle and freehub body laying around that I pulled out of a brand new Ultegra hub for a Fixxer conversion, so you can raid my parts bin for a small fee.
If it's in a wheel, the easiest and usually cheapest option is to buy a new hub and swap the guts; axle, cones, bearings and freehub body. If it's not in a wheel, it's hard to get the freehub body off. Take it apart and see what's wrong. I've got an axle and freehub body laying around that I pulled out of a brand new Ultegra hub for a Fixxer conversion, so you can raid my parts bin for a small fee.