perforated metal lamp diffusers?  

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Unless you are lucky and can get a good one from an otherwise trashed lamp I think you would need to have one made up. It shouldn't be too hard as you can certainly buy perforated metal sheet in all sorts of thicknesses, hole diameters etc. A bit of cutting, shaping and soldering and you are in business

Peter
they are definitely still produced as a bunch of new Nessen lamps were purchased at my workplace and they have them on them. I have looked for them myself online and have not been able to find a seller. You can find white acrylic diffusors online but obviously not the same look. Please provide a link if you find them. I will do the same.

Go it
here is a pick of the diffuser on one of the Nessen lamps at my plase. Note that it is just a flat disk and lacks the rolled edge but close enough I would say.
On the Nessen site there is a link to an illustrated buying guide which I have attached. Pretty sure they refer to this diffuser as a "Downlight Disk". On page 4 of the buying guide there is a section titled replacements parts and it is found there. Largest diameter available is 10.5 inches.
http://www.nessenlighting.com/pdfbig/Nessen%20Buying%20Guide%20-%20Effec...

Excellent. Thanks jdip!
L...
Excellent. Thanks jdip!
Looks close enough to me.