The fashion icon, who is known for donning items from her wardrobe more than once, procured the new accessory 10 days before the July wedding celebration of her husband Prince William's cousin Zara Phillips to Mike Tindall, the celebrity Web site said.
The duchess, herself newly married to the prince, wore the hat with a cream Collette Dinnigan French lace dress and Day Birger et Mikkelsen brocade coat she was seen wearing in 2006.
"She rang and asked to come in, then she tried a few [hats] on and we went from there," Foster told People.com.
"She wanted something in a color to match her outfit," she added. "She had her outfit and her bag with her. ... There were people in the shop but she just strolled in -- she's a normal person, I guess. She's very down-to-earth and polite."
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