How would a person purchase a complete S&W 986 7 shot 9mm cylinder.

S&W won't sell to the public and indicates that they need to be purchased from an authorized distributor but can't provide a distributor.

You might call Midway. They have an assortment of "special order" parts on their website. Perhaps they could order you one.

Keep in mind that the cylinder will need fitting to your gun. It's not a "drop in" assembly.

__________________
Ret. LE, FA Instr, S&W Armorer

The only way I know to get another cylinder, is to send them a 986 without a complete cylinder and say...fit a new cylinder.
I've sent 929's back for repairs missing parts just to get spare parts. Yes, they will call or email about a revolver missing parts, but I reply...It's the only way I know how to get parts.....silence on the other end of the phone.