You can make your own sharpening tool from dowels and a piece of cloth. Simply choose a dowel that fits perfectly to the serrations without any wobbling, and then wrap the piece of cloth half-way around it.
Hold the cloth in place as you refine the blades slowly and cautiously. Always remember to switch the dowel to match the size of the bevel as you sharpen. It is also necessary to choose the appropriately shaped dowel for the serrations – a round dowel for round bevels, and a triangular dowel for V-shaped bevels.
You can find low-cost dowels at any local hardware store.