I saw some at citimart in Hanoi Towers
next to where they sell the razor blades

I can recommend a fabulous place for professional facials - the one associated with Masion des Arts: Yin Yang Spa. I've had lots of facials from some quite high end places in Melbourne, yet this is the best facial I've ever had - in Hanoi no less, and for literally 1/8 of the price!
They do a mud mask for you, using a French brand - I can't remember the name. It's possible you could also speak to them about sourcing this product directly from them.

Find them at:
Yin Yang Salon & Spa
21 Van Mieu Street
email: yinyangspa@gmail.com
Telephone 84 4 213 9283
Website: www.maisondesartshanoi.com

And, if you do try them and don't think they're as wonderful as I do, please let me know!

Here is what Khumbu is referring to:

http://newhanoian.xemzi.com/venue/show/2174/Yin-Yang-Salon-and-Spa

Indeed I was and I've now created an interim review for them as well.

I just went to Vincom Towers and they have heaps of cosmetics stalls on the bottom floor. Siam Spa brand and Le Occitane both do good mud masks... Try once a week not once a day otherwise your skin will dry out though!

Hmmm, maybe hold off on Yin Yang Spa - I've just had a rather negative experience there (and will be updating my review).
Facial I had this time was not half as good as the first one I had (only just passable), and they have real issues with the notion of privacy - I had a procession of random people come in and gawk at me as I lay mainly naked on the massage table (still a good deep tissue massage - they just claim it's by a physiotherapist when it's not). Similar when I was getting the facial.
I'm reverting to doing my own at home.

Answering my own question here - I found a beauty shop at 95B Pho Hue that sells large packages of the powder that you mix with water to make the mask paste.

They bags of powder are on the floor directly in front of you as you walk in the shop. There are all different kinds, and most of the bags have English translations on the back so you can tell what they are. I used mine today and it was great. Only cost 45,000.