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.

You can also buy Biscoff cookie butter. Which is absurdly delicious if you like the cookies.

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.

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.