Baby binet mattress? Where to find one?

Hi I bought an Eddie Bauer binet and the mattress is very thin. I can't for the life of me find a mattress that will fit in the binet. The measurements are 28.7 length and 15.7 width. Is there something I can put under the mattress to make it thicker? Is there a website where I can get a mattress of this size? Please help. Thanks