I haven't tried the kit you mentioned, but the Cortisol EIA StressXpress kit (Catalog Code SKT-201) works well with any sample species and has been validated for use with fecal extracts, urine, serum, plasma, saliva and tissue culture media.
It has a higher sensitivity (17.3 pg/ml) than the Demeditec kit as well.
These products are available in Brazil through the distributors Genese Produtos E Diagnosticos Ltda. (http://www.gen.com.br) or Sellex Inc. (http://www.sellex.com)