I recently purchased a Silhouette Portrait, (currently on Amazon for 170.99) which I love, but I am not a huge fan of the cutting mat. And they are pricey to replace.
When I first posted about getting a Silhouette, a helpful commenter said to use the Cricut mats. I found several people online that noted you can use a regular Cricut mat in a Cameo, but no one seemed to know about the Portrait with Cricut Mini mats.
I went ahead and bought the two pack of Cricut Mini mats ($11.99 at Michaels), and I'm happy to report, they work.
I taped the edges with shipping tape into the quarter inch mark on both sides (so that the rollers didn't hit sticky). And when I loaded the mat, I used the button for the thicker materials rather than the regular mat load button. Works beautifully. Lining up is a little trickier but you will get the hang of where to place your paper/design.
I love the Cricut mats, because they are thicker, less chance of cutting through the mat, which I've heard a lot of people have done with the Silhouette mats.