The adapter that came with the Firefly things that were tested at the LED Museum was rated 1 amp (1000mA) not 300mA not 500mA. And the firefly things they tested sound like the same version OP has (which I don't think they make anymore). The adapter at Walmart that I posted a link for, will work. 500mA may be okay, I wouldn't go with only 300 mA. That adapter should be in stock at any local Walmart. Lol, hopefully, mine has been bad about running out of stuff lately.
edited for typos and such