Sir,
Here's a link to DC think tanks,
SIL DC - List of Think Tanks.
Of those that are high powered on the Iraq issue (based on who news organizations go to for quotes, interviews, and reports or who you see cited most frequently), the following are the think tanks that fall into that category:
Brookings (Michael O'Hanlon is their go to guy)
AEI
CSIS (Center for Strategic and International Studies - Anthony Cordesman is their go to guy)
Rand
SSI (Strategic Studies Institute - the US Army War College's think tank)
Here are some watchdog organizations that don't comment specifically on Iraq strategy but do have an effect on strategy:
Human Rights Watch: Middle East and Northern Africa : Iraq