Lots of options Andrew:RST has 5s and 6s and is happy to mix and match case lots that go for $85.
Eley Game and Target 2 1/2" comes in 5s (US 6s)and is ALSO #85/case
ARMUSA has lots of 12g stuff.
GameBore 2 1/2" 1 oz in English 6s (US 7s) ALSO $85/case
PolyWad Vintager 1 oz & 7/8 oz 6s are $75/case
Polywad 'Vintager' http://www.polywad-shotgun-shells.com 800-998-0669
Eley http://eleyshotshells.com 512-821-9953 or
http://www.kirklees-guns.co.uk/Eley%20Hawk/Eley%20Hunting.htm
ARMUSA 'Vintage' http://www.armusa-performance.com 281-381-7773
Kent http://www.firearmservice.com 512-821-9953
RST http://www.rstshells.com 570-553-1651
B&P now offers F2 Sub Sound and MB Light in 12g. http://www.bandpusa.com/ 800-683-0464
Italian 5s are sorta US 5 1/2s