Can I use a non-organic brown rice flour or white rice flour for the substrate?

The amounts of pesticides in anything sold for human or animal consumption are so minute that they don't affect fungal growth, especially not the one of higher fungi. The pesticides used in grain cultivation are mostly highly specialized herbicides and fungicides(for molds).

White rice flour can be substituted for brown rice flour, but healthier and faster growth has been observed with brown rice flour.

See also What is the difference between brown and white rice?