You can look up just about any instruction manual online, but since many products come with a printed copy consider keeping them in a binder for easy reference when needed and in case you ever want to sell the item.
Traditional values weblog Desiring Virtue recommends purchasing a large three-ring binder and a pack of sturdy page protectors. As you buy products with instruction manuals place the manuals in the protectors. You'll probably also want to add the receipt and any warranty cards to the same sheet protector.
If course bulky phone and camera manuals won't fit in the protectors but for those types of items you probably keep the box anyway. Standard consumer product manuals should fit just fine.
When You Get Stuff, You Get Papers [Desiring Virtue]