In Hanoi, they don't sell enduro in shops. Some guys buy second-handed sportbikes or off-road bikes from Thailand, Cambodia, Laos... and sell on the internet. However, they always rise the price double or triple and who knows what the quality is.
You can find some old motor bikes (supersport, crossing, off-road)on the internet. New bike? No.
In addition, some *rich people import super sport bikes themselves but it costs a lot. You can also take a trip to Saigon where they have wide range of big bikes.

There are several places on Khâm Thiên, Tôn Đức Thắng, Tây Sơn, 120-Mai Hắc Đế; 281-Đội Cấn; 120,..202 Phố Huế and so on...

Hanoi is rather small, so you should go and check them all out, in my opinion. The price is on the ebb cause, fuel price is soaring in Vietnam.

You can also surf some Internet websites:

http://choxe.net/xemay/
http://www.vatgia.com/raovat/1536/mua-ban-xe-may.html
http://www.muabanauto.com/index.php?mod=view&cat=37


Good luck.

My husband saw one in a shop (sorry I forgot the name) but the address is 56 Nghi Tam street.
You can also try The Offroad club,they should be able to help you. Address is 200 Pho Duc Chin. I din't have the phone number but it's maybe worth checking it out.
Hope this helps a bit. Good luck!

Thanks, everyone for the advice. I looked at the websites that minhlenoir suggested, and there are indeed a greater selection of motorbikes in Saigon, and not a great variety of larger, used bikes in Hanoi.

It looks like this search may take a bit more time, which is fine, since we are going to be here for a couple of years. I will keep the rental bike or buy a smaller bike until I find what I want.