we got our amby the other day, and the "sheets" are just bits of cotton that have been shaped and elasticised like a fitted sheet - i figure anything that protects the mattress (even a flannelette nappy) would suffice - just make sure it doesn't have any lumps in it when you wrap it around (for babies comfort)

if you have an old cotton or flanelette sheet, chop it down, get someone to over lock the edges and hey presto - cheap cover! the expense of getting the proper "fitted" sheets is a bit exorbitant to be honest - if ours wear out, i'll get some made to be honest - much cheaper!