The stirplate I just bought didn’t come with an extra magnet for holding the stir bar in place when I pitch the yeast (I didn’t expect an extra magnet to be included). So, where can I buy a cheap magnet? Radioshack? What kind of magnet should I get?
Harbour Freight sells some strong magnets.
Look for Rare Earth Magnets.
Something like this:
These particular magnets are pretty small but they have also larger once.
You don’t really need a magnet to hold the stir bar. Just a piece of iron should work…nail, bolt, etc. It won’t be as strong an attraction as another magnet, but I’m assuming your not turning the flask upside down and shaking it. 
I just purchased several from Home Depot. The stick on strips and some larger sized magnets. They had various sizes and styles. They were in the section with all the nuts, bolts and other connectors, but I had to ask where they were. I guess my eyes just weren’t attracted to them. ;D
IIRC, the prices ran from $2 - $6.
Instead of buying a magnet, just buy an extra stir bar. It’ll do double-duty - for the purpose you discuss here, and as a spare in case you lose one.
True. Maybe I’ll just do that!
If you do have Harbor Freight in your neck of the woods, that’s where I picked up a little magnet with a keychain-type split ring already attached. I bet most any hardware store would have them, but at HF they’re about $1. Pretty much tailor-made for the purpose.
Get several.
My money says if you only have one stir bar, it will end up in the beer despite everything you do to prevent it…
Having several keeps this from happening. Just like having one hydrometer. You will break it in a week. By 2 and you can use the same one for years and years.
That’s why I shelled out for a Nalgene lab hydrometer a few years ago! ;D
I’m planning picking up a few extra stir bars at my LHBS next time I’m over there.