Mouser has a nice single ferrule kit, covering 12, 10,
8 and 6 AWG, in a nice organizer containing lots of ferrules:
www.mouser.com 288-1032-ND $79.38
There are twin wire ferrules on the same catalog page.
http://www.mouser.com/catalog/633/1427.pdf
We tried the crimper from ferrules direct:
www.ferrulesdirect.com FD1006L Long Handle crimpling tool $49
It handles 10 through 6 gauge.