http://www.buydubiaroaches.com/dubiaroaches.html
This is the place i'm using. They're in Arizona and they have all sizes. The medium that I get are only $13.97/50 (delivered post paid). The large are like a dollar more. Check them out.
You Can check the forum sponsors. You can also check the classifieds. Sometimes members sell roaches in there and maybe you could work a deal with them