You can buy them in stores. Be warned though that eating them too often can ruin them. They're an awesome random treat but a little much when you have a whole package of them. Learn from my mistakes.
It's actually a specialty from belgium / north of France called speculoos, biscoff being just a brand of that type of biscuits that managed to export them worldwide.
In the north of France where I lived for a while, it's an absolute staple akin to what peanut butter is to an american maybe or Matcha to a Japanese. Speculoos butter is spread on bread, lots of pastries are speculoos flavoured etc.
When you’re in SF, you can go to the Pier and there’s a little coffee shop near the entrance near the aquarium-Biscoff Coffee Corner—where they will serve you one with every coffee drink you buy, unprompted.
In truth the coffee there ain’t great, and the prep is a bit hit or miss, but the folks behind the counter are nice, and you can buy all of the cookies and cookie butter you could want; but it’s probably best not to overdo it. I think they even have a cookie butter latte but never tried it since that sounds too sweet.
