I bought one from M&P on eBay for mine. It is a Superdream one but it has the correct profile rim. Having said that, the rim wouldn't fit because the distance from the M5 threaded hole to the edge is about a mil out, so I used the existing rim. An hour of fettling and I've got a normal looking headlamp with a 55/60W H4 bulb in it. It cost me £31.
Here's the thread... CB750K1 Headlamp unit