dweste,

A good source of free strong (possibly rare earth) magnets is to open up old computer hard drives and remove the plate magnets housed inside. Old server drives are better, their magnets are much bigger and stronger.

As others have noted, they are both strong and can be fragile due to stresses from dropping them or accidently letting two of them jump together.

FYI When handling magnets, make sure that you DO NOT allow any of your skin to get caught between two magnets. A buddy at work that collects these magnets has several blood blisters from getting his fingers and parts of his hands pinched.

Regards,
Comanche7