All the places I know sold out.
You can make your own using Tropic Marin products.
The recipe is here (from BRS):
Recommended Tools for mixing:
100 mL graduated measuring cup
Small kitchen gram scale
Magnetic Stirrer (convenient, but not required)
1 liter Recipe & Directions
Step 1: Starting with a clean container with at least 1 liter capacity, add 750 mL RODI water. Room temp water is ok, but warm water will help dissolve the powder more quickly.