Check Ebay, Gun Broker, Guns America & Guns international. My guess is, aside from maybe Cheddite, it will be 6-12 months before we see new 10 gauge hulls available.
Other alternatives, buy some loaded 10 gauge ammo, take them apart and then cut them down for your use, look for antique brass ones at a gun show. Most can be reloaded using a large pistol primer and I just bought a whole bunch for about $5.15 per hull shipped or look for some at some hulls at gun auctions.
http://www.amoskeagauction.com often has loaded 10 gauge ammo in their silent auction. That's were I got the antique brass hulls.
Some sites to look for auctions: Invaluable, Proxibid.
