I have one GoFit Core disks for sale. Retail (at City Sports) is $25 for one, Amazon has them for $18.xx each.
I saw an infomercial for the Ab Circle and I'm going to use that to get a shapely core.
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EDIT - Even though the title says $20, I can't seem to change it. There is 1 disk for $10.
Both core disks have been sold. Stick to flutter kicks.