Use the following steps to obtain extra ammunition and weapons.

50 shotgun rounds
Climb a fence after your strength is at "2". Press Right + I + II.

50 bazooka rounds
Climb a fence after your strength is at "2". Press Left + I+ II.

10 flash bombs
Climb a barricade when your strength is at "1". Press Up + Right + Run + II.

10 super grenades
Climb a barricade when your strength is at "1". Press Down + Left + Run + II.

50 flame thrower charges
Hold Up + I + II and press Run when parachuting at the start of levels 2 or 5.