Well I didn't have experience but I think that QIAmp DNA stool mini kit will be a good choice (Qiagene) but as for low coast since you are in Iran you can ask the SinaClon Company for more economic kits but of course with lower yield. (http://sinaclon.com/en/?page_id=269)

Thanks a lot for your help

paper of B.W.Zhang et al ----Extract DNA from feces without kit

Title----A Widely applicable protocol for DNA isolation from fecal samples

if you are loking for a really cheap solution- try the organic extraction, combined with 95C heating. but it wont allow you to deplete all inhibitors. For higher quality DNA isolation try the the PureLink microbiome DNA purification kit