A 10:1 is about as thin as I`d go for a first mirror. Do it in plate unless you are going to repolish it in a few years as most firsttimers usually get only a 1/4 wave mirror unless they are really trying to do an excellent job.
This means a 1.25" for the 12" is the better thing. The secret to doing a good job is to not use pressure on the last grit or the polishing/figuring and to make sure that the glass is turned often while grinding and polishing/figuring. Astigmitism is usually the result of not doing that turning of the mirror and tool often enough.
There is a guy that advertises on Astromart and Astronomy Mall often that has plate glass blanks in many sizes and the 10"x1" size is one of them. I just looked and the ads aren`t listed at the moment but he`ll probably be up within the month.