lol this happened to me. I used a small bolt. I cut about 1.5 inches off the end. I stuck plastic drinking straw over the cutoff piece of the bolt and shrunk it with a lighter.
I called it my ghetto stir bar. I used it for two starters. No, it probably wasn't food grade (although maybe, it was a drinking straw), No there were no plastic or metallic flavors in my finished beer (it was ed's pale ale and turned out delicious), and yes I am still alive today.
Why i didn't just give up and shake my starter like bcstuff says above is beyond me. I think it's because I had just finished the DIY stir plate project and was determined to get the damn thing to produce some yeast. In my mind waiting for a stir bar in the mail was not an option. Neither was shaking lol.
Up next: Kolsch, Alpha Squirrel, Tripel rebrew, Witbier
Currently Fermenting: Charlie Sheen (barleywine)
Kegged: Tiger Blood IPA, Irish Red, Strict Observance Tripel
Bottled: Dry Stout, Pliny the ..... Squirrel?, Cream Ale