For Oz users, I've just found that Officeworks sell Fuji Xerox A3+
copy paper in various weights: 90gsm, 100gsm, 120gsm, 140gsm, etc.
Go to their web site at
https://www.officeworks.com.au/ and search
for "sra3 colotech".
I checked on the Fuji Xerox web site to be sure of the size, and
they show that paper size as 450x320 (p.3 of
http://www.fujixerox.com.au/general/content_request/paper.pdf), so
folded would give 225x320, pretty close to the B4 that is
unobtainable here. Works for me as I have two printers that can take
that paper size.