I get my metric fasteners from BelMetric [0].
Huge selection including obscure styles and finishes. Nuts, bolts, screws, hardware, etc.
One of the best things about them is that they sell small lots including single screws if that is all you need.
Another great thing is that they ship quickly and if your order is relatively small it arrives in your mailbox instead of needing to be delivered by UPS or another shipper. They ship orders in packages appropriate to the order, unlike Amazon.
They also add products to their inventory if customers need something they don't normally carry.
Prices are also reasonable.
I found them years ago and now they are one of my main suppliers. McMaster Carr is another.