You can find a public instance by looking at our documentation https://sap.github.io/spartacus-docs/spartacus-faq/#is-there-a-demo-of-a-spartacus-based-storefront-available.

It currently points to https://spartacus-demo.eastus.cloudapp.azure.com/electronics-spa/en/USD/

If ever you need to use the backend public instance, look at the network tab, and use the provided server.