You should be able to source Mil Spec OD from automotive paint dealers. If you run into dead ends, you may consider getting in contact with your local National Guard Armory and asking the Motor Pool guys where they get it.
Be careful who you ask!! Had a friend get investigated for treason for taking a paint chip off a vehicle in the navy. There are "Top Secret" radar and heat absorbing paints that the military uses. Homeland security will probing you for a couple years.
I heard a story that some guy stole some radar absorbing paint and sold it to people who wanted to confuse police radar guns.
The military uses coatings that conform to their performance specification called CARC (Chemical Agent Resistant Coating). CARC paint does not absorb RADAR or do anything anything exotic like that. It was meant for chemical warfare to allolw for easy decontamination.
You can get the official colors without a problem from just about any manufacturer, they will have them in their paint chip book. I don't think that you would want to pay for the actual coating because it is VERY expensive, contains VERY nasty components (HDI), and feels like sandpaper to the touch - it is a low gloss paint.
I helped quote a Dodge truck program for TACOM called ComBaTT (Commercially Based Tactical Truck), it was my first introduction to military vehical engineering and all of their requirements. It was quite an eye opening experience.