The best luck I've had is the camera store on Hang Trong, south of Nha Tho. The owner is there sometimes, and really knows his stuff. If he doesn't have something, you can usually order it.
However, I've found much better luck in Bangkok, at Panthip Plaza.
The one on Hang Trong is this one:
http://newhanoian.xemzi.com/en/venue/show/1718/A-Dong-Photo-Co
and I second bethanybauman's suggestion that you try there.
The last time I bought anything like that it was in Bangkok, mainly because the selection is better, but also because photo gear is cheaper in Thailand than it is here.
I hear there's a shop on Tran Hung Dao St that deals in photographic supplies (paper, chemicals, assorted developing accessories) so that might be a place to try too.
