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Beaducation is one of the best resources for stamping supplies. They are pushing the metal stamping envelope more than any other supplier. They
do have a selection of gold filled disks.
Halstead, Rio and Rings & Things are among the places that carry the most GF findings. R&T does not have GF disks. I didn't check Rio,
The link to Beaducation's items is
http://www.beaducation.com/shop/index.php?main_page=index&cPath=2_24Halstead has a smaller selection at
http://halsteadbead.com/Product.asp?PassVar=Gold-Filled||Charms%20and%20Drops||Tags,%20Sequins,%20and%20Blanks#ProductTopI couldn't tell if either of their items are single clad (Gold on only one side) or double clad (gold on both sides but only half as thick a layer of gold on either side). Since it is not specified, I suspect these may all be one sided GF.
You can use a disk cutter to punch out your own disks from GF sheet. You also can use metal shears to cut free form shapes.