Good news! I found the larvae in my compost heap!
Now the tricky bit, how do I set up a compost heap where Black Soldier Fly is the dominant species? I put the larvae I found in a bucket with some avocado peels, pumpkin skin, etc., but so far I'm getting mostly fruit flies. Do I just need to be patient, or is there something I can do at this stage to encourage BSF/discourage other species?