You are in the U.S.?
Most stores like
TSC carry Permethrin Poultry Spray or Dust. I prefer to buy concentrate to mix up and spray the coop/roosting bars/nesting boxes and use dust for the birds themselves. But you can also use the spray on the birds too if you wish.
I put the dust in a sock, it helps me control the dust. I tap the sock onto the birds back, around the vent, under wings, etc., releasing the dust, then rub/work the dust down into the feathers getting it to the skin. If using spray, you'll want to part feathers and get that worked in too. (This is why I like dust better for the birds, easier for me to work with).
For the coop/housing, it's best to remove bedding and dispose of it. Spray the roosts/nesting boxes, nooks and crannies of the coop - places where mites hide.
Treat the birds and the coop in 7 day intervals to break the lifecycle. I'd put minimal bedding back in the coop until you get rid of all the mites. Depending on how heavy an infestation, then you may need to repeat several times.
Unfortunately it seems DE, garlic and other remedies are not that effect against and infestation. These may help as deterrents when birds are bug free, but once the critters take hold, then you have to step it up to Permethrin.
You should be able to find similar products at your feed store. The concentrate will make many gallons. You can also buy premixed as well, but it's not quite as economical.