You can find tartaric acid sold online or in speciality shops, such as those specialising in confectionery or home-brewing.
This didn't work out for me. I followed the method, yet, all I got was a mayonnaise-like consistency. I had to use much more than 1/8 tsp. I will try this again, though. Thank you for the recipe. - 21 Jan 2017 (Review from Allrecipes US | Canada)