Where to find this in HCMC?

As far as I have been able to find out, there is no "Arm and Hammer B Soda" in Vietnam but baking soda is easy to find at any large market. The occassional supermarket has it as well.

Thank you. Which is the main supermarket in Saigon that sells baking soda?

u can find at Hon Phong 1175 - 3/2 st -D.11

I think I've seen it at the An Phu Supermarket in D2

Saigon has ten million residents so there are many markets big and small. Ben Thanh Market is in District One and is known to all xe-om drivers. Ba Chieu Market is in Bin Thanh District and is also known by all the xe-om guys. Whichever District you live in, it will have several markets, usually one big and several small. Xút is the word for soda in Vietnamese but be sure to say Bi-car-bon-ate preceding Xút or you could end up with Caustic soda. In some cases I hear them say "Soda" with the emphasis on the "da" and they are pretty smart at figuring out what you want. If you write Bicarbonate of Soda on a piece of paper, somebody at the market will help you find it.

You can go to Thai Ha Shop on Ham Nghi street, very near to Bitexco Finacial Tower. Sure you can find it there.