Finding a good power bank that can charge the Nintendo Switch whilst you continue to play is tough, but finding one that’s a reasonable price is even tougher.
There’s currently a good deal on eBay for a RAVPower (one of the better brands) Power Bank with a capacity of 20100mAh. A power bank of this size will charge your iPhone about 7 times over and your Nintendo Switch about 4 times.
Heading to JB Hi-Fi, the a USB C power bank of the same capacity will set you back about $169, so you’re doing pretty well to grab it.