A 12VDC computer fan from an old PC or something else being tossed, 2 neodymium magnets glued across from each other near the rim of the hub the fan blade is attached to, in even a cigar box works wonderfully. Power it from a 12VDC wall cube (don't go higher - these things don't like being abused much).
If you want a speed control search the web as there are already designs to do this which are simple (very few parts). Home beer brewing enthusiasts apparently grow their own yeast beyond the first batch so they don't have to keep buying the expensive pre-packaged yeast and they need a magnetic stirrer to do it and most go the computer fan magnetic stirrer route as its quite inexpensive and easy to build where several of them have documented their designs and put that information up on the web.
One thing though. These fans all kind of stall around 5-6V on the low end so whatever you'd use for a speed control will only have the fan moving above this output voltage up to full bore at 12V.
You want the thing in a sealed box as that creates enough static pressure that it seriously reduces drag on the blades and the thing will spin faster. You don't need to go breaking the blades off the hub as some suggest.