I get some OEM from South Sound Honda (use the 1976 parts listings), some from CMSNL, some from PartsNmore in Canada (Ontario?), some from VintageCB750 (seems to be the same as PartsNmore?), and used parts from either this forum or eBay.
For carb parts: stay with OEM. Do NOT use aftermarket carb brass parts, like from Keyster or K&L, in these carbs. You will find a nightmare of bad/no performance, among other problems. You can get jets from JetsRus.com, if you need them: your mainjets should be either #110 if made before 3/72 (frame date) or #105 after that.