Before looking for where to buy borax online, maybe you should know where it's commonly used.
Borax, given the E number E285, is used as a food additive in some countries but is banned in the United States. In consequence certain foods, such as caviar, produced for sale in the U.S. contain higher levels of salt to assist preservation.
Its use in cooking ingredient to add a firm rubbery texture to the food, or as a preservative. In oriental cooking it is mostly used for its texturing properties.
In Asia, Borax (Chinese: 硼砂; pinyin: péng shā) or (Chinese: 月石; pinyin: yuè shí) was found to have been added to some Chinese foods like the hand-pulled noodles lamian and some rice noodles like Shahe fen, Kway Teow and Chee Cheong Fun recipes.
In Indonesia it is a common, but forbidden, additive to such foods as noodles, meatballs and steamed rice. The country's Directorate of Consumer Protection warns of the risk of liver cancer with high consumption over a period of 5–10 years.
Now where can I buy boric acid? Well, they're available online.