Yep,
Apparently they haven't stocked the 25.4mm Borofloat in the past, it was ordered in for some guy in NZ and they now intend to keep some on hand. Leslie assures me they can supply cut to size as you demand, so go for it.
I believe it is almost identical to the Pyrex in terms of strength, weight & expansion, thermal shock resistance etc.
I couldn't find any other place in AUS that stocks it apart from this mob. (I don't have any connection with them in just to clarify). I'm just letting folks know since it is often hard to track down this type of stuff because there isn't a big demand etc.
Good luck with your mirror!
Cheers, Fred...