This is where I bought the gongs.
http://stores.ebay.com/The-Widget-Shop-Inc?_trksid=p4340.l2563They have all kinds and all sizes.
You can choose the thickness of the gongs and of course the thicker it is, the higher caliber it is rated for.
You can buy 1/4" all the way to 5/8" thick.
Here is a great link explaining the hardness ratings.
http://www.agarms.com/pdf/steeltargets.pdfThe guy I bought them from ships via priority mail and is great about shipping all in one box. (he's not trying to profit off of shipping)
I bought a 4",6",8",10" & 12" and he shipped them all in one box for $15.00
I paid 19, 23, 50, 60, & 90 for the gongs.
The 4 & 6 are 3/8", the 8 & 10 are 1/2" & the 12 is 5/8" thick.
I bought the angle iron from Home Depot. For 5 pcs it cost me $70 (enough for 2 frames)
The chains and links cost me another $50.
Five cans of spray paint cost $30.