Thanks for your replies (and sorry for my very slow one – it's been a very busy holiday season).
Regarding locally sourced mirrors, I called ~10 glass and mirror companies around Brisbane (and then some of the companies they suggested trying), with no success. In most cases,
I had to explain to
them what a front/first surface mirror is. Two companies did have FSM, just not what I'm after. They both had it only on 2mm glass, and couldn't guarantee quality (one of them said waves would be visible under magnification). One mentioned that their FSM are for use by hobby artists in kalaidascopes; they import direct from America. If anyone is still interested, the 2 companies are Hartley Williams & Co Pty Ltd, Brendale and Albion Glass & Mirror Co Pty Ltd.
Conclusion: Fail.
This left me with overseas options:
Frank,
SURPLUS SHED did have some first surface mirrors, but nothing big enough to do the job with a single piece of mirror, as is my intention.
I contacted firstsurfacemirrors.com to ask about the optical quality of their 1λ and λ/4 FSM, and this was what they had to say:
Generally, our standard 1/4" glass mirrors work great without blurryness. Our 1/4" glass mirrors (1λ) are clear up to 15x magnification and the λ/4 mirrors are clear up to at least 60x. The cost of small λ/4 mirrors isn't too high but a large mirrors, such as the 12" x 12" you mentioned would cost $900. The price calculator for these is here.
Since they won't guarantee their standard 1λ FSM past 15x magnification (I was hoping to at least get a mirror guaranteed to work on the next size up from my current binoculars (i.e. at least work on 20x80 binos), since I'm paying the extra to
import mirror). It is quite possible (according ot another expert opinion I received) that it
might work, but I need someone to guarantee it before I fork out the money. And the λ/4 mirror is just
way too expensive.
Conclusion: For anyone else out there looking for FSM, this company is still definitely an option to consider.
I contacted Vacuum Coatings Ltd. in the UK (thanks for pointing me to them, Michael)
and they have wound up being the winner. My interaction with with their Terry Pearce was very confidence-inspiring (he's the other 'expert opinion' I mentioned, above). Both on
this page and in his correspondence with me, he guarantees his FSM to work with 20x80 binos
provided the piece of FSM has been personally selected and optically tested (i.e. you need to tell them that you need it to work with 20x80s when you place an order). This really inspired confidence for me – knowing that someone who knows their stuff has personally optically tested my piece of FSM and is guaranteeing it to work with 20x80s. He selected a piece for me that is 8 1/4 x 12 inches and is charging me £36, plus £24 p+p (£60 total = ~AU$100). I'm happy, and am proceeding with the order.
Thanks again for all your input. I will plan to post pics and my experience when I have it all put together!
) with the intention of making a binocular mirror mount (guided by 
