To answer the original question, I don't believe there are true leather pads that will fit the Portapros, unless you make it yourself. Even if you made them, the sound may change and have a negative sound impact. It'll probably sound a little muffled compared to the stock foam pads.
IMHO, the portapros are super great headphones for the money $30-50, but if you want leather ear pads on a headphone that's relatively light and portable, look into the ATH-ESW9/A, but you can buy 6 Portapros for just one.