You might want to go back to some of the earliest techniques used to
isolate single bacterial colonies -- sliced potatoes.
Before agar, before gelatin, the realization that single bacteria made
single colonies came from potato slices "gone bad." You can sterilize
them fairly well with a pressure cooker (not too long, or they will
crumble).
Expose one for 15 minutes, cover and keep warm for a few days, and
you'll have both bacteria and mold. With luck, you will have single
colonies, the result of a single bacterium reproducing to form a large
group of friends.
You could try exposing for different length of time, in different
places, or adding small amounts of pond water to seed growth. You
might want to dilute the water with sterile water (pressure cooked)
to reduce the number of bacteria.
I'd recommend covering with something that doesn't touch the potato
surface, but keeps the potato moist -- an upside down glass on a plate
might do.
