Where can I buy a replacement roku usb power adapter

You may have one around from a phone or some other device.  Just be sure the fine print on it says its output is 5 volts and at least 1 amp.  Higher amps is okay as the Roku will only draw what it needs.

USB power adapters are very commonly available from many retailers that sell all manner of household or electronic goods, so you can probably find one locally.

You can also purchase directly from Roku: https://www.roku.com/products/accessories/roku-micro-usb-power-adaptor