Customization items can be found as dropped loot, in vending machines, or as challenge/mission rewards. They can also be traded with other co-op/online players. To unlock a customization piece, go to your ECHO (menu) and find the item in your Inventory/Backpack. Select it to “use” it; the item will now be shown in any Quick Change machine per the designated character or vehicle it is for. Simply use 10 customization items between all characters to unlock the achievement.