They’re probably quite rare now, which is why you might be struggling. I did briefly consider some on a previous MX-5, but they were heavier than the available alloy wheels.
Is there a particular reason you want/need steel wheels? As you could just use 16" alloy wheels with winter/all-season tyres fitted?
I’d look at a set of ND 16" wheels, or you could choose a 16" wheel from the NB, e.g. The SVT 16" wheel. I saw this set available for £100…
No real reason they had to be steel, just thought it would have been an easy/straightforward affair but you’re right. Seems easier just to snag some alloys and fire on some winter tyres.
I’ll keep an eye out for an old set of NBs and failing that get some new alloys. I assume the ones in the image aren’t available