Most good federal agencies will have a study guide. Currently in the process for the USSS and they have one that from what I have heard from others is spot on for the actual test. The feds always want you to dress up, local agencies could care less. I wore a suit to a city test and everyone else like you said, was dressed in band t shirts and ripped pants.
Most state, Sheriff’s office and city agencies will also have study guides. If there isn’t one on the website contact the recruiter address/phone number and ask.
Test material between fed and state/local is comparable. One was not harder than the other (currently Fed LEO) and the content and structure was virtually the same. As far as dress code goes, I applied with probably half a dozen local agencies in addition to federal agencies when I got out of the military. I NEVER saw anyone underdressed at a state/local test day or hiring process. I’m really surprised that happened, I couldn’t imagine going to any prospective job related event and not being dressed in at least business casual.
Thanks for the replies.