I purchased ring magnets from this supplier: http://www.carolina.com/p2p/endecaSearch.do?keyword=magnets
I use them as counterweights on my XT8i. They are sold in a set of 16 magnets for $4.50 plus $6.95 for S&H. I purchased two sets and the S&H was still $6.95. Each ring magnet is 1 3/4" diameter and 16 magnets together weigh 21 ounces. I have had no issues with the magnets other than the fact that they are very strong, so be careful not to pinch your fingers.

Dan