There are very few power banks that support USB Power Delivery 2.0 right now. The ZMI 10 is one, Ravpower has one too but I can't recall the model number. In fact, I would expect PD2.0-capable power banks of this capacity to be all around $60-100.
For practicality, most Switch users I know off just bought a normal 5V/2A power bank. This is enough to charge the Switch when it's asleep, or continue to power the console when it's low on battery and "dong" till they reach home, but it's unable to power and charge at the same time. Since the Switch doesn't seem to draw no more than 2A regardless of voltage, the only way to have both power+charge happening at the same time is to go higher than 5V (hence PD2.0), something most power banks don't currently support.