Dear Questioner

I have worked at Neal's Yard Remedies in Oxford, England; a city where there is a constant nit epidemic. Neal's Yard sells the bark you mention under the name 'quassia'. It is a very bitter substance. Many parents find it very useful. It comes as wood chips which need to be boiled for half an hour, left to cool and then strained ready for use as a hair rinse or spray which is left in the hair until the next wash. In each pint of water, use one ounce of quassia chips. I think it is possible to mail order from Neal's Yard in the States, which is where I assume you to be.
Kind regards, Anna