The main way I get reports of BSF is from people with traditional compost bins. BSF frequently show up in them and it usually makes a big impression on the unsuspecting gardener. \:\) Most of them do a little research and then embrace the BSF larvae.

You can ask people who compost in your area and also feed stores. Where I live people call BSFL "meal maggots" because they're often found in open or wet feed. If you don't know the feed store employees they may not be inclined to admit the occasional presence of fly larvae, even wonderful flies like BSF.