Where can one buy anti-UV umbrellas in Santiago, Chile?

You can by an umbrella that blocks UV in virtually any store that sells umbrellas. This is because black umbrellas block roughly 90% of UV, and at that point you'll need to start worrying about reflected UV rather than direct exposure (Ref: https://jamanetwork.com/journals/jamadermatology/article-abstract/2597893).

If you want to get from 90% to 95%+ with a black umbrella, hold the umbrella you're considering up and look at a light through it. The less you can see light through the fabric, the less UV will pass through.

Finally, choose a large umbrella. Not only will this reduce reflected UV from nearby walls hitting your face, some have two layers of fabric.

When travelling you will often find that the ways things are marketed at home are not applicable in other places. In this example, perhaps people in Santiago are not explicitly concerned about the UV blocking of their umbrellas or are somewhat aware opaque umbrellas block UV. By contrast, in cities in South Korea or parts of China you would find tons of umbrellas marketed as blocking UV, because that is a main use case.