I think creating one's own test and changing it every year is the only way out for these schools. Otherwise it isn't fair. This whole thing is bonkers.
Although--is it possible to prep for IQ tests in the same way? The content is very closely guarded, right? I assume it isn't as much for this ERB test and that's why it's so out of hand?
I don't think there is much difference between the ERB and IQ tests. Any test that becomes an entrance exam will be prepared for.
Amazon has a book for <$10, "Testing for Kindergarten: Simple Strategies to Help Your Child Ace the Tests for: Public School Placement, Private School Admissions, Gifted Program Qualification" by Karen Quinn. Chapter 4 is titled "The 7 Abilities of Highly Successful Kindergarteners: What every test measures and what your child needs to know: language, knowledge/comprehension, memory, visual, spatial, cognitive, and fine-motor skills". I don't have the book, but the index lists many pages for both Wechsler and Stanford-Binet.
I consider using a book sold openly on Amazon to be fair game. Test publishers would sue the book publisher and/or Amazon if the book revealed trade secrets. There are probably sites and people, especially testers, who go further than this and cross the line.
