I tried the trash bag method I mentioned earlier and I really didn't like it. It's too hard to get the bird in the bag, and it's still messy.
I don't like the powders at all because it's so easy to get them on your skin or inhale them. So I looked around, and I found
Poultry Protector. I bought it from Jeffers, but it's available at a lot of places. The active ingredient is potassium sorbate - a food preservative, and completely safe for humans. (It's in a lot of stuff we eat!) I bought the concentrate instead of the ready-to-use spray.
The bottle says to spray the birds, but I found that it only kills the lice if the liquid directly contacts them. Spraying just didn't work for my birds' infestation, so I created a bath for them instead. I mixed up the solution as directed in a small Rubbermaid tub and gave each bird a bath, making sure to work the solution into the feathers so it would reach the skin and the lice. It worked great! You do have to repeat it several times over a few weeks to kill any lice that hatch out.
I highly recommend it, and I won't use the powders again.
If you do want to use potassium sorbate (Poultry Protector), I suggest looking around for a source for potassium sorbate. It's available online because it's used so widely as a preservative, and it's a lot cheaper to buy potassium sorbate in bulk instead of under the Poultry Protector brand.