Resources

trailofbits.github.io/ctf/forensics 

In a CTF context, "Forensics" challenges can include file format analysis, steganography, memory dump analysis, or network packet capture analysis.
picoctf.com

Pico CTF has a large area dedicated to forensics in particular.

root-me.org

root-me is  a large security platform, with a extensive section dedicated to forensic challenges.