Substrate for house flies is dry dog food diped in milk. Put in a couple of adult house flies in the cage & in a couple of weeks you should have larvae. There should be a small container with moist potting soil in the cage for full-grown larvae to climb in & pupate. Also pupae can be removed & put in the mantid's cage before the adult house flies emerges.
Greg, have you try to call them? Skip e-mailed me saying that he may still have some pupae left in the inventory. Try to e-mail him again at [email protected]. I have been totally dependant on his house flies all this while.
Yen, I did not even try because on theire website they said they would not be taking on NEW customers. But I may give it a shot. Thanks Yen and Brandon.