Finding housing here is generally limited to Airbnb and two active Facebook groups where listings are posted every day. Prices are quite reasonable overall but expect to pay a little more with Airbnb.
Housing for Expats in Hanoi
A reliable expat-focused Facebook group with accommodation listings from individuals and brokers.
Hanoi Massive Housing
Active Facebook group for both expats and locals, which means you may find some cheaper deals, but will have to sift through a lot more postings.
Airbnb
Plenty of monthly listings that are easy to search and prices are not overly inflated.
Alpha Housing
With good quality apartments and houses for rent, this real estate agency caters to foreigners looking to stay in Hanoi for awhile.