Ingredients

In which area of the country do you want to find those items?  If it's Saigon:

1-  Castile liquid soap can be ordered in various volume (imported from the UK) online at: http://nguyenlieulammyphamhcm.com/nguye … -soap.html ;
or in person at 122 Nguyễn Văn Khối, P9, Gò Vấp District ;
or over the phone at  01682 101 038 or 0974 295 698. 
They'll deliver COD on the same day to any place in Saigon.

2-  Borax is available practically everywhere.  The name in Vietnamese is hàn the.  The staff at your accommodation can point you to the nearest neighbourhood market, or if you want to purchase a large volume, head to Kim Biên Market, 37 Vạn Tượng, P13, D5.  You can find borax at every stall in the chemical section.

3-  If you meant baking soda when you said washing soda and sodium carbonate, it's available at every supermarket, every mom-and-pop general store, and every household item stall in every neighbourhood market.  The one in my pantry right now is from a Korean company called Only Price and sold at Lotte Mart.  Arm & Hammer baking soda is sold in more expensive stores such as Annam Gourmet.

Thanks for that. Washing soda is different to baking soda. It is a cleaning agent.

How about Zinc oxide?

Oh and bentonite clay

I see the difference between baking soda (NaHCO3) and washing soda (Na2CO3) now.  Na2CO3 is called soda ash in Vietnam.  I've found a few places that sell it, but the product is from China and I'm leery about chemical stuff made in China.

I wonder if there is a local substitute?

Large volume (25kg) of zinc oxide from Thailand can be purchased at http://hoachatsg.com/tan-phu-trung/zinc … -thai-lan/ ;
or in person at 539/35 Lũy Bán Bích, P.Phú Thạnh, Tân Phú District ;
or over the phone at 028.3860.9465 – 028.3860.6917 - 0932.522.166 – 0909.952.166  – 0902.728.288;
or by email at [email protected]

You can find bentonite clay at spas, or shops that sells spa salt, spa soap, spa masks, etc.   Here's one:
http://nguyenlieulamsoap.com/d-t-set-tr … -clay.html

Here is Na2CO3 from India:
http://dactruongphat.vn/san-pham/soda-a … co3-an-do/

All the rest (there are pages of it) is from China.

You're the best! Thanks