I understand pre-mixed concrete bags (including the stones) are not available here. I'm not a concrete/cement expert so I would like a recommendation (Brand/type) for the best alternative cement bag mix to use to build a simple retaining wall, stacking individual unopened bags, as in the attached picture. I have mixed plenty of concrete for other projects around the house but I am looking for a simpler solution for this project. I am aware of the need to add drainage points behind and through the wall, especially as my ground is mostly rocky clay. 

You can buy rice bags for 5 baht each.  Then buy SCG red structural cement by the bag and a cube of sharp sand and a cube of blue metal.  Hire two blokes for a day to dry mix the ingredients and put them in the bags and then stack the wall.  Make sure you have a big tarp incase it rains before the bags are stacked into place.

16 hours ago, Silencer said:

I understand pre-mixed concrete bags (including the stones) are not available here. I'm not a concrete/cement expert so I would like a recommendation (Brand/type) for the best alternative cement bag mix to use to build a simple retaining wall, stacking individual unopened bags, as in the attached picture.

Since the picture has dry concrete (cement sand aggregate) and is significantly more expensive than the individual items you can’t find it here. The pictured wall while easy is much more expensive than the volume of concrete it contains.

 

That said you could just stack bags of cement they will harden but have not much structural integrity or strength, they will also be vastly more expensive than the idea of filling rice bags with dry mixed concrete.