Instead of a normal street level accessible store, why not a vehicle somewhere (on a map without too many triggers & such) that acts like the van to the suburbs (or Arbor's car). Instead of taking you to the suburbs it just takes you to either a blackmarket armor shop or implant doc?

Access would be through Victor. Perhaps add a completed mission counter and after so many missions if your character is 'elite' options to request a visit to the black market armorer or mafia medical clinic would be added to his dialogue options.

If an eligible player selected one of those two options, they would get an appropriate 'ticket' mission item that would enable use of the vehicle as a portal and delete the ticket from the player's mission items when used.

Possibly could restrict access to every x missions completed by resetting Victor's counter.

And yeah the above probably sounds a lot easier to do than it actually would be, but hopefully it wouldn't give much opportunity for bugs to be added to what works now.