They are
native and abundant where you are. Any sizeable
compost in the open, especially one containing food scraps,
should have plenty. They are the big "maggots", often over an inch in length.....housefly larvae are smaller. The adults look rather like a black wasp, and wave their antennae up and down when they
land......
In a large
enough compost, they should overwinter even as grubs, especially if you mulch the pile. In any case the mature larvae crawl out and go dormant in the soil, to hatch out and appear next spring. This characteristic is what makes the various bins work so well for them, since the mature grubs crawl off on their own into a handy collection container......