Pool alkalinity products

[Lynden]

Found the answer this morning at Font Arboleas. The Astralpool PH+ is pure granulated Bicarbonate of Soda, perfect.

[Lynden]

Everywhere can sell me ph+ and ph-, but my pool has an alkalinity reading of just 50ppm. I need something to boost this above 100ppm. So far I have been unable to find anything in any local pool store. I can use good old bicarbonate of soda ( baking soda), but where can I buy it in bulk?
Failing that is there any pool store that can help that I have missed?