Basics- Neodymium magnets are the best, small disc shapes will probably suffice.
Around 1/8"X3/4" ones should be great for securing armor to a suit, remember, though, their power decreases exponentially with distance, so make sure the fabric between is pretty thin.
Sources- I'd try Ebay, or just google a retailer.
It's also possible to use old hard drive magnets- isn't it two per hard drive? Regardless, you have to rip the drives apart, but they are neodymium and very strong, a thick spacer might be a good idea to make the adhering force less.
Safety- the only pacemaker danger sign I've ever seen was at the NHMFL where they have 9 Tesla electromagnets with liquid nitrogen cooling and other assorted ridiculousness, there was a clearly marked radius o' death on the concrete. I don't think small magnets used to hold armor on are life-threatening, but keep any pockets away from adjacent fastener sets.