I attempted the Sisyphean task of finding a stir bar locally. I just couldn't justify paying for shipping for a little $3 bar. In the end, of course, I did pay the shipping.
If you have any kids in high school chemistry class, maybe they can borrow one for you. Otherwise, northernbrewer.com. G'luck.
Oh yea, one more thing. I have a 4000mL and wouldn't get one smaller. Especially if you want to boil your starter wort inside it. I don't think NB has that size.
Reason: ... oh, one more thing...