Salvador's often has them, as mentioned above, and the fruit stand on Hongshan Dong Lu (虹山东路) next to the Anquan Xincun bus stop (安全新村站) always has imported Mexican avocados.

The passion fruit stall on Hongshan dong lu imports its products from Myanmar, during avocado season (summer months) they regularly have them. They might be able to put you into contact with importers for bulk purchases. However, now is not the harvest season so will be difficult.

Chilean avocados at Wal-Mart for 7.9RMB a piece.

I asked 3 fruit sellers why they offer only Mexico (50y kg, about 10y for piece) or Chilean (40y, about 7 for one) and they said the taste of avocados from Myanmar is not good, even is cheaper! I found the quality of Chilean is may best in carrefour, price 7.8 a piece...if price of Myanmar one is cheaper，hope I can find some during the seasson..
Salvador offered them for 16y a piece...

One of the day-time vendors on Hongshan Dong Lu is currently selling avocados from Myanmar. 30 yuan a kilo.