I spotted these and thought I'd have a punt - Qty 20 for £10 inc. postage - and they've arrived today in the post.
https://www.ebay.co.uk/itm/BOX-OF-20-x-CAST-IRON-PULLEY-WHEELS-MULTI-PURPOSE-ROPE-CLOTHES-AIRER-WHEEL/192902362046?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
As the description says, they were intended for clothes lines and are (I guess) obsolete hardware stock. But they are cast iron castings and can be turned (pun intended) to different uses. They are just under 44mm in total diameter with a thickness of 10mm. I think (for use in G3) the maximum scale tread possible would be about 2ft 8" (bottom of pulley groove is 37mm) so you could probably get a turned tread of 36mm, one side of the pulley wall acting as a flange. So not big enough for standard gauge wagon wheels but I know some here also work in other scales! (16mm for instance).
I will be using two of mine for a small job I have in hand and perhaps others will be used as 'rollers' in a rolling road test rig I've been meaning to build. At 50p each, they are cheaper than ball bearings for the test rig and will work just fine for that kind of work I think but obviously this may involve a bit more lathe work....
Anyway, just a thought in case they are useful to others here too :-)
Regards,
IanT