A quote from the article where it says not to use it no more than once per week as it is so abrasive.
"Dr. Hani Saeed, a dentist and a professor at Dhamar University, believes the miswak has benefits, but can be over used. “I am against the daily use of the miswak because it can grind down the enamel of teeth,” he said. “If it is used no more than once per week, the stick is fine, but I discourage people from using it any more than that.”
“Using toothpaste and a toothbrush is more effective than the miswak,” said Dr. Saeed. “The brush can reach every where in the mouth and kill bacteria between the teeth.”
“I always clean my teeth using the miswak after every prayer during Ramadan month. It gives me a fresh and good breath, and I feel better to have clean teeth and healthy gums,” said Malika Farah, 22, student at Sana’a University"
I see them in Indian grocery stores. I never had any luck with them so I gave up.