Try CMSNL in Holland. They have now got CB750 systems that for a long time were NLA. Im not sure if they have got japan making more or they are repros but what ever they are, they are much needed for restorers as unlike 2 stroke systems the 4 stroke ones rust out. So they may have 500 ones available fresh stock or NOS you never know. Other places i would try are Econo Honda in NZ and David Silver Honda Spares in UK. Both seem to have a large supply of NOS Honda parts. If that fails you could get them made. I know a guy in SA who can make them. Ive had him make a 750 system plus many others