Commercial flat pack furniture is often joined using special nuts and
bolts. The nut is a piece of metal dowel with a threaded hole drilled
at right angles to the axis. This goes into a hole in a flat panel and
the bolt (usually with an oversize head) goes through another panel
then through a hole drilled in the thickness of the first panel into
the nut

The stength of such a joint always surprises me. Does anyone know
where I can buy such fixings?

Dave