Hmm
Take some tank water and add to QT as well as some filter media?
Don't know if that suffices.
Also, planning ahead, maybe a small stone / rock thats been in a container of tank water by a windowsill with direct sunlight for algae growth onto the stone / rock may sustain oto for food as well?
Thats all can think of tbh, not really something I've thought about, lets hope some experienced folks may come up with some tried and tested methods.