Go to eBay and do a search for 'Rare Earth Magnets'. You should be able to find cubes (that's what I prefer) in sizes ranging from 3/16" - 1/4" (bigger as well, but your not going to hang you plane from an I-Beam are you?) Actually, I've seen 2" cube REM's that can handle 200+ lbs and then cost about $40-$60 each!

I've had very good luck with this seller.

http://stores.ebay.com/Emovendo