"Is there a public list of ACLU donors?"
I have no idea. You'd have to ask the ACLU. I'm sure they have such a list. I'm equally sure that they're not going to give it to you. If, however, you are a marketing firm, you might be able to buy it.
The ACLU doesn't really need "donors". They make a living on "civil rights" cases for which they are paid by the federal government via USC Section 1988. Our tax dollars are used against us by the ACLU.