Hmm, I think if a salon uses a certain product (say shampoo, styling products, etc.) chances are the product will be available for sale through them.
I think the last time I went to Monsoon ID my stylist did asked me if I need to purchase any styling product or not; but I don't style my hair so I couldn't be bothered

So I'm guessing the clay can be bought from them, I have no idea about the price they're selling though.
On the other hand, you can try getting the Sebastian clay from
K Care Shop.
Based on the prices listed in their website:
50g - RM 48.00
125g - RM 90.00
I am unsure if these prices are reasonable or not though

You can try checking their 'Store Locator' to see if there's any of their outlets near your area. Then you can personally visit the salon to check it out

Here is the website:
*CLICK*p/s: I am not affiliated with the K Care Shop and I actually have no experience buying anything from them; it's just that I so happened stumbled upon the shop's name when I was researching about the availability of the clay in local markets.
I bought my Sabastian at around RM 50 (or RM 55?) for the 50g one, therefore the price of RM 48 is incredibly reasonable! The 50g one can last at least one year and if you don't style everyday it can last longer.