You'll find the most ornate newels at salvage yards, starting at around $150. Choose one that's about the same height and type as the newel you plan to replace. The one below is a "rail-to-post" type, meaning that the handrail secures just below the newel's finial top.
To attach the newel, you need specialty fastener kits. At the base, use a two-way Sure-Tite screw ($16 at Stair Parts USA
), which has wood threads on one end and machine threads on the other. Drill a pilot hole in the framing of the stair where the newel will sit, and twist in the wood-thread end. Drill a corresponding hole in the bottom of the newel, and fit the post loosely over the fastener’s projecting machine threads like a sleeve.
The handrail is fastened much the same way using a rail bolt ($4.40 at Stair Parts USA), which has wood threads that go into the newel and machine threads that go into the rail.
To tighten the assembly, twist a washer and nut on the machine thread end of both fasteners. Access these through drill-outs in the side of the newel and underneath the rail, and hide the holes with wood plugs.