I have purchased leather loincloths from Ebay ... a few were used in films and were from the Western Costume Company. There is a seller who I have a link for under the marketplace section of the site called Debbie's Leather who makes leather loincloths. (I guess the Indian Trading Company is no longer so I'll have to remove the link.)
One thing you can do is connect two smaller sections of leather (the front and back flaps) with a brown fabric thong sewn to the inside top of each flap to connect them so as the loincloth can be worn over a belt or tied at the waist. This is how most of the Western Costume Company loincloths were made and they were comfortable and felt authentic.
Hope some other loincloth fans respond with some suggestions and thanks for being supportive of the site.