Another place to get the 12.5 ruler, pretty much always about half price or less is Wawak.com (no affil).

Personally, I think I'm going to check out my local hardware store and see how much it would cost to cut some plexiglass and how accurate they are, because I like the idea of having many sizes without the ridiculous and silly sewer's price markup.

This is how I have always squared blocks.

You can also check local glass shops to have one made.

I say bite the bullet and buy the 12.5" square up ruler, too. I tried all (except going to the hardware store for plexiglass) and NONE worked for me. Even though I tried to square up my own, I always ended up with a skew. Yeah...12.5" this way and 12.5" that way.....but they all leaned. I tried with different rulers, different mats...... ARRRRGH What I found with the 12.5" SUR I just bought......it has all kinds of markings on it that will help with ANY quilt block size smaller than 12.5".......

What a great hint! Never heard of this place before.

Right now they have the 12.5' square for $15.00

[TABLE="width: 100%"]
<tbody>[TR]
[TD="colspan: 2, align: left"]Omnigrid® Omnigrip Ruler 12 1/2" Square[/TD]
[/TR]
[TR]
[TD="colspan: 2, align: left"]Omnigrid® Omnigrip Ruler 12 1/2" Square for quilting, sewing and crafts.[/TD]
[/TR]
[TR]
[TD="colspan: 2, align: left"]
[TABLE]
<tbody>[TR]
[TD="align: right"]Product:[/TD]
[TD]Omnigrid® Omnigrip Ruler 12 1/2" Square[/TD]
[/TR]
[TR]
[TD="align: right"]Stock #:[/TD]
[TD]TA32[/TD]
[/TR]
[TR]
[TD="align: right"]Brand:[/TD]
[TD]Omnigrid[/TD]
[/TR]
[TR]
[TD="align: right"]Unit:[/TD]
[TD]Each[/TD]
[/TR]
[TR]
[TD="align: right"]List Price:[/TD]
[TD]$26.99[/TD]
[/TR]
[TR]
[TD="align: right"]Our Price:[/TD]
[TD]$15.00[/TD]
[/TR]
</tbody>[/TABLE]
[/TD]
[/TR]
</tbody>[/TABLE]