I just discovered a medium that's odorless (well is smells lightly like hay) and should be an excellent gutload for mantises...GUINEA PIG POOP!
Granted this requires you knowing someone who has guinea pigs or owning guinea pigs, but this way you can control what the maggots are eating. a) guinea pigs are always eatting pollen rich hay and B) whatever fresh greens and fruit you give them. just don't feed them carrots very often if you intend to use their droppings as medium. I just mix it with a bit of water for moisture. You can also use guinea pig urine but that makes it a little stinky.
Advantage of this also is it's pretty darn odorless, so in the winter months you can still culture flies if you have to keep them indoors. without the smell of course =) and it's free...well for me it is. And if you own a guinea pig, just train it to use a litter box, they can be trained.