Amazon! Diatomaceous Earth Food Grade 10 Lb https://www.amazon.com/dp/B00025H2PY/ref=cm_sw_r_cp_api_3DnBAbKZHNXTF Super cheap, and lasts a while, I've had my bag for over a year now.

I've heard that Diatomaceous Earth works amazingly well for getting rid of pests. I would give that a shot first.