they sell scooters (bikes & parts) but also run a bike rental shop with some Minsk. They may be OK to part with one or two Minsk, ask them.
Or you could contact by phone Cuong's Motorcycle in Hanoi (his shop and phone number are listed in LP) and ask him if he has partner in HMC who have Minsk for sale. I've heard that his brother runs a shop in HMC, where rented bikes taken in Hanoi can be dropped off. Not sure about this, but it's free to ask him.
Have a nice ride, this is a great trip. I still have my Minsk stored at vietnamese's friends' house, waiting for my return in 2007 to visit the Mekong Delta and Cambodia.