Try McMaster-Carr. Virtually everything is available in US and metric units. There is less variety of things like metric fasteners, because designs have improved to use fewer weird parts.

Since almost everything is designed in CAD and manufactured by CNC these days, the real distinction between US and metric units has practically vanished. A dimension is a floating point number.

Most stuff is at least secondarily labeled in metric units. In many cases the metric measurement is actually the authoritative one in the specifications and contracts, even when it's in smaller print or parentheses on the package.