Source for engine hardware?

What is a good source for engine hardware? Nuts/bolts/washers, etc?

If you have the original bolt in your hand and you're looking to buy a new one, you can measure the length, thread pitch, identify the bolt's grade from the marks on the head, etc... but is there a source that can help identify what you need if a given bolt has been lost, sheared off, etc...?

Thanks!!