just to let you know I have moved this thread to general discussion.
For the bow you can use any useable material, plastic rod, brass rod, steel rod (as David rightly suggests), the old metal wire coat hangers you get from the dry cleaners will probably do the job, the broken off end of a fibreglass fishing rod, in fact anything that will keep the fishing line under a reasonable tension and give a bit of curve so you have a reasonable hold at one end. As for using it to polish pivots, what sort of chuck does it have, i.e. how big an arbor will it take or are you only using this for pocket watch pivots? You may find you are moving its position up and down in the vice to account for the different size arbors to keep the pivots straight in the wood block.
Just a thought