Unable to find a source for rotary mirrors, how should I search for them?
The items you've posted are all complicated, specialized integrated assemblies. They're also very purpose specific.

As such, I can't think of many places you could just buy them as-is, independently of the device they're intended to work in.

Anyways, I do know that most laser printers to indeed use such a device, though I'm ~90% sure that the devices in the picture from the OP are all from bar-code scanners.

Basically, in a laser printer, they're used to sweep a modulated laser across the photosensitive, electrostatically-charged drum. The charge on the drum is affected by the light striking it, and then becomes patterned, affecting how toner adheres to it in the next step.
Anyways, in such a situation, the laser is only swept in a line, so the aspect-ratio of the mirror is small, so they're typically very short.

In the mirror assemblies in the question, the mirror drums are very tall. Since the height is not needed in a laser printer, they're probably from a bar-code scanner, that needs to sweep the laser in an x-y range.



Anyways, short of a few OKI printers (which are billed as a "LED printer" anyways), anything sold as a laser printer should have some sort of rotating mirror apparatus inside it, though how much luck you'll have making it work outside of the printer itself may vary.



Personally, I would check places like All-Electronics or ebay, if you can't track down some old laser printers locally.