Mostly on-line. Google "rare earth magnets" and you'll see a ton of options. There are some sellers that seem to be the favorites of various detachments, but for me it's about quality and price. For example, Radio Shack sells "rare earth super magnets" on line, but I wouldn't buy them. Jo-Ann's used to sell two good quality sizes in-store, but they are still available on line: http://www.joann.com...ound/xprd74968/ and http://www.joann.com...ound/xprd74521/. That said, there's no one seller that I always buy from unless I'm in a jam, then I'd hit Jo-Anns.
Absolutely! I E6k'd my magnets and let the helmet sit for at least 24 hours before playing with it and admiring my genius handiwork. I did try coating them with PlastiDip to protect the magnets (these guys are so strong that sometimes they shatter when joining) and to add extra strength, but it was unnecessary and just pulled off.