Try Mathew at Telescopes & Astronomy in Adelaide, they list blanks and grits etc on their website or try some of the larger glass suppliers, they may cut you a disc of whatever size you want. I remember a fellow club member got a 15" X 1" blank from somewhere in Sydney a few years ago, sorry I don't remember the name of the supplier.
Know anywhere in Australasia one can aquire 30mm or thicker float glass or borosilicate plate glass? I know of suppliers in China that supply out to 70mm thickness but the problem is the minimum order thing,ie like 10 ton or 5000 pieces or 1 container lots. I be interested in grinding a 30+ inch mirror.Currently working on two 22" f5.2 mirrors and one 25" f4.6 mirror but stuck with 19mm float only. Mirror support could be an issue with the 25". I know it can be done as James Dobson went out to 24" on 19mm thickness for a workable mirror and I know of two 30"x1" mirror dobs that are ok.