I get food grade/CODEX DE from my feed supplier. It comes in 50# bags. If is not perma-guard, but it is food grade and it has the CODEX on it so I trust it. I just hassled her till she found a supplier for it. I think more than the name brand what is important is the fact that it is food grade/CODEX level. Other kinds of DE can contain insecticides and other ingredients harmful to people and chickens.
I mix mine with the feed at 2% by weight. It is used as a preservative in foods, anyhow, so mixing it in their food is no biggie. I do it in a wheelbarrow before storing it. I also sprinkle it on the floor when I clean the coop and under the bedding in the nest boxes.
While we do have flies this year, they are nowhere near as bad as I'd expect considering how wet it's been here in Missouri. I mean, you're gonna have some no matter what you do. It certainly cuts way down on them, though.
No parasites, no mites, no worms, no lice...I'll keep using it, thank you very much.
Trisha in MO