Recently, I checked every store in a local mall and only found two stores that carried women's shoes in sizes larger than 10: Payless and Sears.
I had better luck online and here are the stores I found that have larger sizes in women's shoes:
BarefootTess.com: up to size
DesignerShoes.com: up to size 15
dreamshoes: up to size 16
DSW Shoe Warehouse: their search engine goes up to size 15, but it found no women's shoes in sizes 15 or 14 and only sneakers in size 13. The selection started improving at size 12 and smaller.
Endless: up to size 16
Gwyneth Shoes: up to size 15
outletbuy.com: up to size 17.5
Payless ShoeSource: up to size 13; I have bought a lot of shoes from Payless (online and in store) and some of my purchases are the most comfortable shoes I own.
Zappos: up to size 16; I have made two purchases from Zappos and have no complaints. They are fast and they pick up the tab for shipping in both directions!
(Thank you Paula for a correction and an addition.)