Ya know...you can make your own starter from just flour and water. There are lots of native yeasts floating all around and everywhere and they will make a home in your starter mix. What's nice about using a commercial starter instead, is that the yeast is already in full production. The army of yeast helps to combat any foreign bacteria, or other fungi from taking hold.
You can also start your starter using a little packet of baking yeast with your flour and water. It won't be wild, but it will do the job.
I think that using the grape could be a good idea. I believe that grapes have a layer of yeast growing on their skins.