For posterity, found a battery solution that worked great, iof anyone else has the same problem -- https://www.amazon.com/gp/product/B01M7Z9Z1N -- has a built in switch, has 12v out via a barrel jack, has 5v via USB. USB stays on as long as the switch is on, even under tiny load (just an Arduino). I linked it to a barrel jack adapter, here: https://www.adafruit.com/product/368 . Power to Arduino via USB, common ground to the barrel jack power adapter terminal block, capacitor across the terminal block, power and ground out to the Neopixel strand.
17v and 800 mA is an unusual value. Since most 12-volt supplies are usually more like 14 volts, one of these 12-volt rechargeable battery packs will probably work fine. You will need to work out a comptible cable to connect the ampifier, though.