I've used Fastenal before, but usually rely on a local joint called Shamrock Bolt and Screw.

Either one is great for sourcing specific fasteners when you know exactly what you need.

And on more than one occasion I've brought a random bolt or screw in to Shamrock, handed it over to the person behind the counter, and they've disappeared into the back only to return a couple minutes later with exact duplicates.

Has come in real handy for times when (for example) one of the screws for my knife pocket clip fell out, and I had no idea what exactly tiny little screw was needed to replace it. Or when a specific oddball bolt for my custom wood platform bed got lost while moving. I just bring in another one and have them match it.