What have they been eating up until now? I had mine on house flies and crickets at that point I think. The crickets varied in size based on how long I kept them and how big they got (I bought them as "small" crickets but I didn't use them regularly so the last of them were pretty big when I finally used them). For the crickets, I either removed or damaged the jumping legs to make things easier for the mantis.
I also think I have read that L5s could eat blue bottle flies but I didn't try it myself. Someone else can probably confirm that.
They should be able to eat Blue bottle flies. I have fed my L5 bottle flies before. House flies work as well, though they are a little smaller. And crickets too of course.