I was just about to suggest Princess Auto.
I think those 8mm ones might be a bit small.... We've got a whole bunch of those ones on our fridge at home. Wish I was there and I would test them out....
http://www.princessauto.com/en/detai...ts/A-p8183907e
These ones on the other hand, I struggle to get off the fridge... I bet these would work. Maybe even too strong lol. I made the mistake of using one to stick a couple receipts on the fridge and it was so stuck to the fridge it let me remove the paper around the magnet... but I couldn't get the magnet off.
Otherwise it's princess auto... just return them!