I use Alvin mats, purchased from Amazon. I think they're just as good as (and VERY similar to) an Olfa mat, but less expensive.
Don't forget to hydrate your mat periodically! Take it into the shower with you! Mine is too big to fit into my tub, so I lay warm damp cloths over it a few times a year to rehydrate it. That will help keep your mat pliant and self-healing longer.