It's called a "stir bar" and I'd suggest eBay, where you can probably find one for less than $5 including shipping. The more usual sources would be retailers like coleparmer.com and fishersci.com.
You can put together a stir plate for less than $30 though (link
). It might be worth it, especially if your pump has the plastic head. Eventually you'll just strip the threads.
Also, if memory serves the magnets in the March pumps surround the impeller. I'm not 100% sure they'd turn a bar that was a few inches removed from the hub.