Instructions
Beat the game without alerting any soldiers throughout the entire game. However, this is a very difficult task and can be over-looked and one can obtain the Stealth Camouflage with other means.
Beat the game normally. After the game is completed at least one time, many new features are unlocked during the next play-through of the game, such as the ability to buy the Stealth Camouflage.
Obtain Drebin Points. One of the best ways to obtain as many Drebin Points as possible is to find an area from where soldiers are constantly coming, such as the Vista Mansion in Act 2. Kill them and sell what they drop to Drebin for a lot of points.
Use Drebin's shop and purchase the Stealth Camouflage. It costs 5,000,000 Drebin Points during the first four Acts and 2,500,000 Drebin Points during the last Act.