What exactly do you search for? I've tried looking but I have no idea what to search for.
https://the-eye.eu/public/MSDN/Windows%207/
- en_windows_7_debug_checked_build_dvd_x64_398741.iso
- en_windows_7_debug_checked_build_dvd_x86_398742.iso
https://the-eye.eu/public/MSDN/Windows%20XP/
- en_windows_xp_professional_x64_chk.iso
EDIT: if you want to get the actual build, I can't help. I only remember it being available through MSDN AA back in the day.
I don't recommend chk builds for routine use by anyone outside the Windows teams because even if the regular Windows release still receives updates, the chk build typically doesn't. CDN disk space isn't cheap, and neither is CPU time on the build or test machines.