Snaggletooth.
The great what shall we wear bible does not permit the wearing of Chip Bags (or Forage Caps) with No 3 Dress (CS95) - obviously unless you are DCINC STC
Strictly Jungly
You can purchase them from any good military tailor - we inherited them from the Army. Many regts/corps still wear them in barrack dress (although the wearing of barrack dress seems to be restricted to Shrivenham now with the rest of the Army wearing their combat pajamas)