Patches/Insignia
AV8RStuff.com - Aviation patches gone wild. Lots of specific cruise patches if you want to be reminded of a "be-no" cruise.
BCPatch.com - Some vintage stock and fair pricing. Former submariner has all the U.S. submarine patches.
LJMilitaria.com Lee-Jackson Militaria sells original vintage patches and other collectible military gear.
MarinePatches.com - USMC aviation insignia galore. Kind enough to let Bluejacket.com add some missing squadron patches.
McGrogans.com - Chief McGrogan carries an outstanding stock of small unit insignia - brown water Navy specialist.
Military-Graphics - Jim, USN retired, turns any patch or military emblem into a vinyl decal for a reasonable price
PlaneCrazyEnt.com - Excellent selection of reproduction aviation patches
RexWess.com  - The best selection of Coast Guard items is from Rex, USCG retired.  Check his book on USCG patches.
TheBattleZone.biz - All services, a bit of everything and well organized.
 
Posters
PostersInc.com - Reproductions.  Main source for the recruitment poster dealers.
Rare-posters.com - Nice originals at a price.

Pictures
DefenseLink.mil - Latest images and news releases from DOD. URL seems to change weekly.
Defense.gov/photos - Similar to above site, one of a number of ever changing URL's in government.
archives.gov  - National Archives and Records Administration.  Patience will yield amazing results.
USNI.org  - Naval Institute picture library has all U.S. Navy ship photos back to 1900 for a fee.
National Naval Aviation Museum - Buehler Library holds over 500k U.S. naval aviation photos and on request will provide a limited number of digital scans via email.

Note:
Most patch/insignia and poster dealers stock reproductions. Original items are rare but occasionally may be found.  Watch out for highly padded "shipping and handling" fees of some auction dealers. If you don't see the shipping fees posted it is best to verify shipping and handling fees before you buy.