Diatomaceous Earth is not a shampoo. Your thread title and the word "shampoo" confused me a bit. If your birds already have lice, you can try to get food grade Diatomaceous Earth (not regular DE!), but you'd best get poultry dust or Sevin Dust and nip it in the bud quicker. DE will kill lice, yes, but it's expensive and harder to find than the other. Pet stores also carry a lice/mite spray for parrots if you have only a few birds.
ETA: When I bought the late great Hawkeye, he came complete with a bad lice infestation. All I used was food grade DE in his bedding in the quarantine dog kennel and on him and it killed them all, so it does work, but if you have more than a few birds, it may be best to use something else if the lice have a foothold.