Try contacting Lyndex. They make a wide variety of 5c collets and will
probably have a 6mm and 12mm square, and the 5mm hex.

http://www.lyndex.com/prod_standard.asp?type=r85c3j

Sprydle wrote:
I'm having difficulty locating 5c collets specific to what I'm trying to
do.

I have 2 different sizes of square brass stock. The first is 11.85 mm
square (.467 in) the second is 5.96mm square (.2345 in) which (if I'm
right equates to 15/32 and 15/64 respectively (can you tell I only do
metric? :-) )

I can't find collets in these sizes - or should I be trying to get
metric collets? Since the metal was purchased in the US, I think I
should be trying to get English sizes.

My other collet is for brass hex stock - 4.98 mm across flats (0.196
in) which I think equates to 3/16. I have managed to find hex collets
of that size so I think I'm ok there.

Any help appreciated!