If you really can't find any off the shelf, you could try making it yourself. The reagents are cheap and common -- anhydrous citric acid (powder), water, and sodium bicarbonate (baking soda).
e.g. https://survivorshipnowvt.org/how-to-make-sodium-citrate/
I see other instructions online for making it with sodium hydroxide (lye). Don't try that unless you have a hot plate, pH test strips, and safety gear like goggles, gloves, and a lab coat. Lye is strongly alkaline and fairly dangerous at high concentrations.