There's a mat that Joann's sells (or used to - I have 3 of them), that easily fit an 18x24 cutting mat, pressing board and 24"+ rulers and tons of other ..... stuff.
They were around $30 but with a coupon......
But I can't find the link....It's blue and I think it was a Dritz or Omnigrid(?) product.
eta:
All I can find are nylon ones - mine are canvas or duck cloth.
An art supply store would also have portfolios that would also work for you.
Anyway, you can easily modify any bag pattern to make one for the size you need, with the pockets you want.
http://linda-stokes.blogspot.com/201...s-holiday.html
http://www.supermomnocape.com/2012/0...-zippered-bag/