Hello!
You should be able to find porter bags at many places. The biggest collection would be KURA CHIKA Kyoto shop, on the 2nd floor in Fujii Daimaru department store. It is where Teramachi st. and Shijo st. meet.
http://www.frontstyle.com/shopsearch/shop_detail.php?shopid=S_001639
I remember seeing their bags at Takashimaya department store, and at LOFT(shopping building).
They are all close together, so you can easily go the three on foot.