My 14 year old Olfa is my goto mat. And I use the 18-24 the most. It fits on most tables, whereas it's hard to find space for the 24x36. I have no idea if Olfas are as good as they used to be, and hope I never have to find out.
But if I did have to find out, Wawak.com is another source. They always offer a fairly steep discount and they sometimes run sales over those prices as well.