Are you looking for a timing pulley (i.e. a pulley that you might use with a belt), or are you looking for a winch pulley (i.e. a pulley acting as a spool to wrap a rope or wire around)?
If you cannot find a pulley with the correct bore size, one work around is to get a pulley with a smaller bore and then drill it out to the right size. Alternatively, you could get a pulley with a larger bore, then mount it using one of our Universal Mounting Hubs for 6mm shafts, which might require drilling some mounting holes in the pulley.