Heres a video on making 2 1/2" shells from 2 3/4" shells
https://www.youtube.com/watch?v=gHYAltIRL98Also apparently Berreta Australia
http://www.berettaaustralia.com.au/c/4469945/1/shotgun.html still bring them in ( But I cant confirm that ). That link will take you to there Shotshell Ammo page, you may want to sift through it, and see what you can find, or send them an email.
EDIT : I did check out Cabelas in the USA, and yeah Winchester and Remington both make 2 1/2" shells Along with a few other brands I havent heard of before. So you could also email Winchester Australia, and Raytrade Australia ( Remmington Importers for Australia ).
AUSTRALIA
RAYTRADE PTY. LTD.
P.O. Box 4212
Victoria, Dandenong 3164
Australia TEL: 011-61-39-706-5936
FAX: 011-61-39-706-5805
Contact: Gary Raymond
And there is a contact form for Winchester Australia mate
http://www.winchesteraustralia.com.au/contact/default.aspx