Plumbers merchants, not sure if they ever did 6mm microbore copper, but must be olives if they did…
Failing that, look for a generic fit Thermocouple for an old boiler. They have a small bore copper pipe, dont know the dia though, but worth a shot…
Motor Factors, might have some for odd car brakes, but most are flared type anyway…
Yellow Pages, try a local hydraulics stockists…
Pirtek, give them a ring, see if one of their vans is local to you today. (Not as in call them out, obviously, but see if they are attending somewhere local)…
We’re a pretty big Pnuematics mfr & we dont have them (cos I’ve just had a look), nylon air hose tends to be 4mm or 6mm.