Mats are cheap and yes certain dies need certain size mats...But most are under 10.00 with Joann's Coupons.
I bought my whole unit with bonus dies and mats from Nancy's Notions...She ran a reg sale then I got an addition 17 percent off entire order...Plus the tumbler die, mat 10x10 and one of the sqs...Plus the reg package that comes with the machine....It was great...so you might wait and ck her site.